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This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Attack on Titan
aka: Shingeki No Kyojin
This page is full of unmarked HEAVY SPOILERS, being speculation about a story with many mysteries.
Please place theories in appropriate folders, if they apply!

    open/close all folders 

     The Grand List Of Things We Know For Certain: 
Confirmed Theories and facts.
  • All Titans were once human. Most have been fully absorbed by their Titan form, with only the central nervous system remaining as the weak point in the nape.
  • The Titan Shifters' goal was to locate the Coordinate and the Beast Titan. The former controls Titans, while the later transforms humans into new Titans.
  • The Beast Titan is responsible for the destruction of Ragako, by transforming the villagers into Titans.
    • The Titans within Wall Rose were the villagers from Ragako. The House Titan is Mrs. Springer.
  • Eren is the Coordinate, and possesses the ability to control Titans.
  • It is possible for a Titan to regain their mind and human form, as evidenced by Ymir regaining human form after 60 years.
  • At least one other settlement exists outside the Walls.
  • Ymir is a surviving member of a group destroyed by the group Reiner, Bertolt, and Annie serve. She gained her powers by stealing it from them.
  • Titan form is a potential Lotus-Eater Machine, hence their commonly blissed-out expressions.
  • The Beast Titan has not encountered humanity for more than half a century.
  • The King (King Fritz. "Fritz" is his family name or name of the dynasty he belongs to, despite "Fritz" being a German first name) is a Fake King and the real royals are the Reiss family.
    • Krista/Historia's family is deeply involved with the Wall Cult.
  • The masters of the Titans, or the intelligence behind the Titans' actions, is in a faraway location, along with other Titan Shifters.
  • Levi and the man who murdered Historia's mother share the last name Ackerman, suggesting at least a distant paternal relation to Mikasa.
  • Christa is far more important than we realize.
  • Hannes will once again face off against the Titan which killed Carla.
    • Hannes will die fighting against the Titan he was too scared to do so before.
  • In one way or another, Levi and Mikasa are related.
  • Reiner, Bertolt, and Ymir all have knowledge of the Beast Titan.
  • The "Coordinate" is not just the ability to control Titans, it's the ability to Mind Control anyone.
  • Normal Titans eat humans in an instinctive attempt to devour a Shifter in order to return to their human form.
  • Eren is not the Coordinate, because the Coordinate is not a person (It is a power passed from host to host, possibly by devouring the host).
  • Marcel /Berwick was a Titan Shifter, and Ymir became a Shifter via Cannibalism Superpower.
  • Captain Ackerman raised and trained Levi.
  • The Reiss family intend to have Eren eaten, in order to place the Coordinate in a more suitable host.
  • The injection Grisha gave Eren is responsible for his Titan powers.
  • Eren ate Grisha to become a Shifter.
  • Grisha himself was a Titan Shifter.

     Things We're Pretty Sure About, But Can't Confirm: 
Theories that have not yet been explicitly confirmed, but are heavily implied.
  • Titans were artificially created as an external threat that would force humanity to unite. This implies that, at minimum, someone was pulling the strings to begin with.
  • Marco died under suspicious circumstances, rather than a standard Titan attack. Annie was in possession of his missing gear, and visibly hesitates when asked how she ended up with it.
  • When the humans took refuge behind the walls, their memories were altered somehow, possibly using the Coordinate ability.

     Jossed Theories 
Theories that have been proven false.

All of the Titan Shifters grew up inside the Walls, but were subverted and turned against humanity or were raised by human factions to unknown ends
  • Jossed, as their homeland is explicitly stated to be somewhere beyond the Walls. Their mission required them to get inside.
The purpose of breaching Wall Maria in 845 isn't just to retrieve Eren, but rather to expand Titan territory.
  • Jossed. Reiner and Bertolt discuss having to enter human territory in order to search for the Beast Titan and the "coordinate", which they believe might be Eren. Bertolt also explicitly states that once they have those things, they intend to leave and never return.
Titan reproduction.
Maybe they eat humans to chemically modify them in their stomachs, and after regurgitating the dead, they turn into more titans?
  • Jossed: People eaten by Titans are just dead, as proven by the clean-up in Trost.
Dr. Jaeger is responsible for Reiner's, Annie's, Bertolt's, and Ymir's status as Titan Shifters
  • Jossed: Ymir spent 60 years trapped in her Titan form. Reiner, Bertolt, and Annie could also be older than they look, and only consider Eren to be a possible Living MacGuffin.
Titans' eating habits are actually attempts to replace a dead pilot
  • Jossed: Ymir reveals she used to be a regular Titan, and remained in that state for 60 years without aging. It was only once she regained her human form and consciousness that she resumed aging and needed to eat, drink, and sleep.
Sasquatch Titans were what the Titans were originally designed by humanity to fight
  • Jossed: Reiner and Bertolt discuss the Beast Titan in a manner that suggests it may be an ally to their group. At the very least, it is important to their overall goal.
The regular Titans are actually some form of plant life, and the Titan Shifters are made using their extracts.
  • Jossed: Ymir states she used to be an ordinary Titan, but eventually woke up and became a Shifter.
The Titan Ilse met...
...is going to appear again very soon. All the Titans in the forest are running after the Squad towards Reiner, Berthold, Ymir and Eren, so this one will be among them and this time will finally find the Ymir-sama it's been looking for. After which something may or may no become clearer.
  • Jossed with the Episode 3.5 OVA. Levi ends up killing the Talking Titan when it tries to eat Oluo, despite Hanji wanting to keep the Titan alive.
The unintelligent Titans are infants.
  • Jossed. Regular Titans can result from anyone, of any age.
Titans are "born" from the vomited human remains they leave behind.
  • Jossed. They don't digest, so Titan vomit is just piles of gore.
All Titans are capable of speech.
  • Jossed. Eren is incapable of human speech in his Titan form.
The Smiling Titan is a Titan Shifter.
  • Jossed: It seems to have been an ordinary Titan.
The Smiling Titan is Hitch.
  • Jossed.
The Smiling Titan is Moses.
  • Jossed.
The next arc will be Bert and Reiner trying to rescue Annie
  • Jossed. They are shown preparing to head home.
Sooner or later, Eren's going to confront the Titan that ate his mother...
..but only after s/he's turned back into a human.
  • Half and Half: Eren and Mikasa do indeed come face to face with the Titan that ate their mother, but the part about regaining human form seems to be Jossed.
There will be an attack on the human settlement area while Eren is taken away.
  • Jossed.
Levi had a role in Mikasa's parents death
  • Jossed. He was a member of the Survey Corps around the same time Mikasa's parents were killed.
Irvin Smith is the King
  • Jossed: The King is shown in Chapter 54.
The Beast Titan is Eren's father
  • Seems to be Jossed. Grisha knew Shadis and Hannes, and therefore would not be curious about the 3D maneuver gear.
the Captain ackmern introduced in Chapter 56 is the Beast Titan, which would mean that the goverment are the real bad guys in the story
  • Jossed. Captain Ackerman is familiar with common military equipment and tactics, ruling this out.
The girl in Eren's memory is his mother, Carla
  • Jossed: The woman is an unnamed character who befriended a young Historia. Specifically, her half-sister, Frieda.
Eren is not the "Coordinate".
  • Jossed. Eren has the power of the Coordinate, though there appear to be multiple people that possess this ability.
The girl in Eren's memory is Eren HER self
  • Jossed. The woman is a separate character.
Annie and Bertolt are related (her father)
  • Jossed. We've already seen Mr. Leonhart in flashbacks.
Ymir's People know how Titans are made, and may be able to turn them back into humans.
  • Jossed. Ymir stole the Titan's power, and only became a Shifter by accident when she devoured Marcel and stole his powers.
Titans become Titan-Shifters by eating humans who share their blood.
  • Jossed: Titans become Shifters by eating any Shifter.
Historia's dad or kaney ackerman is the beast titan
  • Jossed: Lord Reiss and Captain Ackerman have full knowledge of — and access to — regular Maneuver Gear.
Annie was Berrick's replacement
  • Jossed. Goodbye, Wall Sina reveals she was always part of the mission, which is why her father trained her.
Annie won in the spar between her and Mikasa
  • Jossed: Word of God states the fight was interrupted by the instructors, leaving it a draw.
Before he lived with Kaney Ackerman, Levi's parents/ family were nobles
  • Jossed: Word of God confirms Levi grew up in extreme poverty. His manner of dress and love of tea developed out of a fascination with the nobility and their comfortable lives.

     Theories about Eren 
Theories concerning Eren Yeager, his origins, powers, and the Coordinate.
Eren devoured the Mysterious Woman and that is how he gained the Coordinate.
Chapter 53 has Eren stating "Father...had me..." before it trails off. Many think this means Grisha had Eren eat him...but that would require Grisha to be both a Titan Shifter and the Coordinate. While he is mysterious and certainly possesses knowledge of whatever is going on, Grisha seemed very much an ordinary human and he certainly had lived in Shiganshina long enough to start a family. It's more likely that Grisha injected Eren with the Titan serum, transformed him into a mindless Titan and goaded him into eating the Mysterious Woman, thus gaining the Coordinate power. For all we know, she could have been a Shifter too. The Shifter Trio targeting Eren the moment his Titan powers were revealed hints that it might be something only a Shifter could successfully wield.

Eren's difficulties with his Titan form are a result of being the Coordinate
When one thinks about it, Eren has the most difficulties with his Titan form of all the Shifters we've seen so far. He doesn't have any special powers like Annie's specialized hardening, Bertolt's heat-shield, Reiner's body armor or Ymir's insane agility, and he's almost lost his humanity twice, once while just training. Plot-wise it makes sense that the main character wouldn't have perfect control, but perhaps the in-story reason is because he is both the Coordinate and a Titan Shifter, and the two powers are either incompatible or simply interfere with one another. Grisha made Eren a Shifter in order to protect him — and possibly the Coordinate ability but he didn't, couldn't forsee how the two would adversely react. The argument could be made that the Shifter trio had been trained in the use of their powers for their mission thus, their superior abilities, but what about Ymir? She may have knowledge of the Ancient Conspiracy and history of the world, but she outright states that she barely knows anything about her own Titan powers. Yet she was able to harness her Titan form easily when it's implied the only other time she shifted since regaining human form was rescuing Daz.
  • I disagree. While we can't rule out the Coordinate theory yet, but it seems more likely that Eren has problems with Shifting because he doesn't know what he's doing. It's been confirmed that Annie, Reiner, and Bertolt underwent training before their mission, so that's probably how they gained their unique abilities. Of course, that Training may not have been related to their powers at all, but for now it's a more plausible explanation.

Eren's regeneration "burnt out" during the Raid on Stohess District

During his fight with Annie, Eren starts burning after she knocks him into the house and bashes his face in. It's been shown that, compared to other Titans of his size, Eren's regeneration is slow - this may be because of his youth or because he's not as physically strong in human form as the other Shifters are. This suggests that Titans have a regeneration limit, possibly based on how much sunlight they've absorbed and how much they can store; if forced to their limit they have to slow down or they'll catch fire because of their extremely high internal temperature. Note that, after he catches fire, none of his limbs regenerate for the rest of the fight. It's possible that other Titans simply *can't* continue fighting past the point that they can't regenerate, or perhaps that they simply don't, being more disciplined and experienced in their Titan forms — Eren's Titan Form is simply too bloodthirsty to slow down and consider tactics.

All of Eren's dreams are prophetic in some way, but not the same way every time.
He has the foreshadowing dream in the first chapter, and later dreams of his father injecting him with something, and he's dreaming of being home with his parents while he's in his Titan form. It's possible that every dream he has is connected to either an event that hasn't happened yet, or one that only happened in another timeline.

Eren is headed for a major emotional crisis, as a direct result of his Black and White Morality.
Eren has a very stark view of the world, with little room for moral grey areas or sympathy for anyone he considers evil. As a child, his viewpoint resulted in him brutally murdering criminals with the justification that they weren't human. In his confrontation with Reiner and Bertolt, he echoes this opinion that they aren't human and expresses anger over the very idea that Reiner might be guilty over his actions, accusing him of playing the victim. Ymir even criticizes him for how simplified and immature his worldview is, making it clear she cannot trust him when he has those sorts of attitudes. Considering the dark nature of the series, and the many harsh lessons Eren has already gone through, it's unlikely his black-and-white attitude will be changed in a way that won't involve him suffering a major blow.

Eren will learn to use multiple Titan forms, aside from his usual 15-meter class Titan.
Eren's ability to form partial-Titans may be allusion to this, allowing him to adopt multiple Titan forms later as the series progresses. This might come in handy if, say, Eren needs to fight in a large building without destroying its roof in the process.
  • Sort of confirmed. He's shown to generate two smaller forms during experiments, though each is progressively less controlled.

The "Coordinate" is located in the Yeager basement.
The coordinate is not a person/Titan, but rather the stash of info on Titans. Coordinates typically refers to locations rather than objects, and it is a location that everyone on humanity's side wants to get to- save for possibly the Wall Cult, who will want to destroy it. It would be a matter of who gets there first- the 'liberated' titans or humanity.
  • Half-and-Half? The Coordinate refers to a special power, though information on it may indeed be in the Yeager basement.

Eren originally came from the same place outside the walls as the other Titan shifters.
The backstory of how Eren and Mikasa met goes a long way toward explaining Mikasa, and a little way towards explaining Eren's general pissiness, but the events of that flashback open up their own questions. Most notably: in a crapsack-world story like this, a couple of scared, determined 9(-ish) year old kids could probably kill a couple of adults in self defense if they get the drop on them, as Mikasa did. But a scared, determined kid from a loving family and peaceful background is not going to be capable—physically or psychologically—of tracking down a couple of adults, manipulating them into trusting him, and then coldly executing them without so much as hesitating or showing fear. At least not without being extensively prepped by an adult sometime beforehand, and the portrayal of Annie, Reiner and Bertholdt seems to show that Isayama is fully aware of this. In short: sometime before the incident in the cabin, Eren was groomed and trained to kill other human beings without hesitation or remorse—we have yet to see how or why.

So. What other place to we know of that turns out child soldiers trained and capable of executing other human beings (sometimes in very large numbers), just like bitty!Eren? Oh, and you say those child soldiers also are all Titan shifters—just like Eren? Hmm...

This theory has a number of implications:

  • Somehow Eren ended up inside the walls, in Shiganshina, presumably with no conscious memory of his past before that point, that having been wiped. This happened either with or without the knowledge and cooperation of the rest of the group—i.e. either it was part of the shifters' plan and he was placed there with Carla and Grisha on purpose, or someone (probably Grisha) absconded with him and ruined all their plans—thus forcing them to kick in the wall and invade human territory in an attempt to get Eren back.
  • Eren probably had the shifting ability long before ever coming to Shiganshina—assuming he was given it as a child when the other shifter kids were—meaning that Grisha's injection did something other than give him the ability to shift.

One of Eren's parents is a Titan
  • The injection given by Grisha didn't give him power, it gave him the potential to control this power, instead of being controlled by it. Ymir used to be a regular Titan, but somehow turned back. Who is to say one of Eren's parents didn't do the same thing, and then Grisha figured out how to ensure that Eren would always be in control of himself, and never have to suffer like his mom/dad? This can go three ways:
  • His Dad; well, obviously this would explain how Grisha knew so much about medicine, it's possible that before the Titans came, humans were much more medically advanced, but they lost this information fleeing for their lives to get to the city. He wouldn't even have to be a scientist to figure out how to create whatever he gave to Eren to give him self-control, just the knowledge an everyday doctor has about DNA and cells and vaccines could be monumental, because that gives someone whole new approaches towards problems and would make Grisha consider possible causes of the Titan issue that those inside the city never dreamed of. Also, if Grisha had lived through the slow transformation of humans-into-Titans, he'd know at least a little more than those inside the city since he'd experienced transformation into a Titan.
  • His mother. Not much, all I can say is that it might be possible for Grisha to figure out a "cure" using Carla's blood, and that mention that he saved the entire town from a plague could be foreshadowing that Grisha is a genius, capable of such feats.
  • Both: No evidence yet (that I can think of). Really, I'm just covering all the bases, in case someone else finds evidence towards any of the less-proven options.

Eren is an original Titan
The injection Dr Jaeger gave him wasn't something that would give him his ability to transform into a Titan but a temporary cure or something that would keep his powers hidden and also make him forget about this. This can explain how Titan knew what he had to do to shift into Titan-mode.
  • Unlikely, given how much Eren resembles his mother, which is way beyond generic anime resemblance especially in a series with such varied facial features.
  • Also, it's completely preposterous that no one would notice the healing factor manifesting in Eren, especially him - he has no control over it later in the series and isn't aware of it, so there's no way that he had it as a child and never noticed. It's way too conspicuous.

Eren was injected before the series started.
Something is not jiving with the scene where Grisha injects his son. One of the things that Eren says to Grisha is "you haven't been the same since Mom died!" This implies that Eren has been around his dad long enough to notice changes, which couldn't have been after Shiganshina fell. It's clear from the scene that Eren and Grisha are alone; if this took place after the first episode, there's just no way in hell Mikasa would ever let Eren out of her sight, considering how protective she is of him and how close they've brushed death. So, the conclusions:

1. Carla Jaeger was not Eren's biological mother, but was rather a woman who Grisha married some time after the injection. The serum erased Eren's memories of his original mother so he quickly accepted Carla as his mom. 2. The serum's side-effect also gave Eren a viciously sociopathic Hair-Trigger Temper, which explains how he could kill three criminals at the age of 9 without blinking an eye or suffering any kind of trauma over it.
  • Then how do you explain Eren's resemblance to his mother?
    • Or how Eren was the same age in the flashback as he is after the Wall falls?
      • New chapters provide evidence. The woman from Eren's dreams has an insane resemblance to her, due to the eye motif. Said woman is nearly identical to Eren, as opposed to Carla who had less of a resemblance. Perhaps that woman is an illegitimate child, much like Historia, of the woman and is thus connected to the nobility in that way. Isayama loves to put irony in Eren's lap; he hates Titans with a passion *BOOM* he's a Titan himself AND has the capacity to control them; thus, he can treat them like livestock. It wouldn't be a stretch to say that his resentment for the nobility simply because of their cowardice would have him be connected to them to further do the You Are What You Hate thing.

The nature of the Coordinate
So far, the three Teen Titans' mission to discover "The Coordinate." The audience doesn't really know what that is; However, as in all things Shingeki, Isamiya might have other plans as to what the Coordinate really is.
  • Its actually a place in the world that they have to find which will help humanity in the fight against the Titans...or completely backfire in their faces.
  • In line with the Time Travel theory, it's not a place, but rather a point in history wherein the fate of the world rests. This could explain the dream Eren had in the beginning: he was ready to "jump" back in time, so that's why Mikasa said "See you later, Eren."
  • It could also be a code name for a person. It could be Historia, or at least Historia knows what the Coordinate really is.
  • It could be a huge Nothing, a false hope given to everyone. This could also go with the whole grimdark aspect of the story.
  • Given The Reveal in 50, it could be short for Coordinator, since Eren has somehow managed to control several Titans.

Eren isn't the coordinate they were looking for... this time around.
The reason Annie, Reiner, and Bertholdt spent so long looking for him without finding him is because the Coordinate was supposed to meet them as soon as they got into the city when they were kids. Eren wakes up on the first page, knowing that the Titans are going to break the Wall... but that's all he remembers. THAT is why he's crying for no reason: subconsciously, he knows he's forgotten everything he was supposed to remember, and he's just started the loop over again, all for nothing. Sending his consciousness back in time was supposed to send him back with all his knowledge, so that he could intercept the Titan-Spies, explain everything he'd already gone through in the future and all the knowledge that came with it in order to change the path of history. This has been the plan all along! How different would it all have turned out if someone Annie, Reiner, and Bertholdt's own age found them in the crowd and said, "I know who you are, I'm the one you're looking for, here's everything I know, everything you've done hasn't been in vain: let's grow up and save the world"?

The girl in Eren's memory during Chapter 53 is...
  • A previous Coordinate, granted that the ability is somehow passed from person to person ala Avatar and that Eren retains memories of previous Coordinates
  • A relation of Historia's, as Eren mentions that she resembles her
    • The above two theories have gained some ground as Chapter 54 shows the mysterious woman does know Historia, enough to have read to her and is implied to have been a person she knew very well. She also possess powers to control memories, as she was able to erase all of Historia's memories of her by merely performing a gentle Headbutt of Love.
  • Eren himself, being Ft M transgender — the scene is shown from his perspective, looking into a mirror, possibly suggesting that it is his own memory.
    • To check this one, try comparing it to the image of Eren as a girl on the main page (the rule 63 entry).
      • Their faces look really similar...
    • This could be a bit hard to fit into the timeline though, she doesn't look much younger than Eren and we've never seen him with such long hair.
    • Unlikely; you need to take the room into account as well. Looks more like something you'd see in the wealthier parts of Sina, no? At any rate, it would be extremely out of place in the Jaeger household (and by extension anywhere in Shiganshina).
  • Alternatively. Eren dressed in women's clothing for some other reason.
  • Eren's long lost twin sister, due to the resemblance.
  • One of the people Dr. Jager was referring to when he told Eren "Their memories will help you."
    • Eren's actual mother (not Carla), who might have died for one reason or another — remember "You haven't been the same since mom died!"
  • With the reveal that the Reiss family, it's the real royal family, she might very well be Historia's (remember Eren mistook her for Historia) older half-sister — the Princess. The interest they show in obtaining Eren as the Coordinate might also be related with her simillar Coordinate ability.

Eren and Historia are related.
Grisha was a doctor (well-to-do, educated) who had access to Titan knowledge, and we don't know anything about Carla. It seems like the natural thing to assume he discovered or stole his knowledge of Titans and Titan Shifters, but there's no reason he couldn't have had it all along, or that he couldn't have "married into" the knowledge by marrying a girl from a noble family. It's possible that Eren is also from a noble bloodline through one of his parents, and what Grisha was going to show him in the basement that day was his birthright all along. The reveal would also mark a third time that Eren, in a condition that shouldn't be any different from anyone else of his type, is suddenly shown to be special in some way (ordinary soldier to Titan Shifter, Titan Shifter to Coordinate, poor orphan to noble heir).

Eren will end up eating Annie.
If Hange is really right about a Titan Shifter being needed to be eaten in order for — a) a Titan to return to normal — or b) to gain the powers of said Shifter, then the only way for Eren to use the crystallization to seal the Wall up would be to eat Annie.
  • Expanding on that, Annie could tell Eren to eat her, whether for redemption or spite.
  • Expanding further, Levi and Hange will insist it's the only way, but Eren will struggle with the idea of eating someone, even Annie.

The Coordinate power isn't unique to Eren.

  • Anyone could do it, if they were as fearless, pissed off, and single-minded as he is. It's a function of his deeper self, activated by trying to punch and/or yell a Titan to death.

Eren will die near the end of the series or at the very end.

  • After Eren has pretty much eradicated every single titan out there, in a cruel twist of fate, he will have to kill himself also since he too is a titan. Eren will have spent a good amount of his life fighting titans, and he will die as one.
    • The writer is cruel, but not that cruel.

Eren's Coordinate power created the Walls 100 years ago.
  • The Walls are composed of Titans who were ordered to form the three walls with their hardening ability. The Reiss family was originally possessing the power.
  • It's for this reason Bertold and Reiner don't need to wipe out the humans within the walls by themselves after learning that Eren is the Coordinate. It's because he has the power of "dismissing" the titans forming the Walls, and then the regular titans could devour the humans.

Eren accidentally brainwashed Mikasa into being the perfect fighter with the Coordinate.
A supposition (by Commander Erwin to Dot Pixis):

  • Eren can (as Coordinate) control Titans.
  • Humans can become Titans (evidence: Connie's mom)
Therefore: Eren (as Coordinate) may be able to control Humans.

  • Evidence: in two seperate cases, we have seen Mikasa act to protect her life when she was but a moment earlier unable to:

    • At age eight, Mikasa is ordered by Eren to fight to save him from being strangled by her kidnapper. This scene is accompanied by a flash of green lightning around Mikasa's body and a shot of what appears to be Mikasa's brain. Then, Mikasa engages the kidnapper with such ferocity that she, an eight year old, crushs the handle of her knife.

    • Mikasa, in the battle of Trost, has just run out of gas for her 3DMG and has given up hope, declaring she has lived a good life and seems ready to die to the hands of a nearby Titan. Then, we get another shot of her brain, and suddenly she dodges the Titan's swipes, all the while monologuing mentally at how she is shocked that she isn't letting herself be killed.

  • Events explained by conclusion:

    • The presence of lightning: almost regularly, whenever we see a Titan-Shifter transform, we see a flash of orange lightning. We've seen it with Bertolt at the gates, Eren when he emerged from the Titan for the first time, and when Annie transformed. However, we have also seen colored lightning before: Mikasa was surrounded by green-lightning when Eren ordered her to fight her kidnapper.

    • Mikasa's skill: this is a rockier argument to be made, but you could chock Mikasa's almost superhuman, top of the class skill to Eren's Coordinate ability pushing her on. She was traumatized, yes, but Eren was able to kill two grown man with ease. It's not a perfect argument (she could just be that skilled) but it is an argument that shouldn't be overlooked.

     Theories on the Titans' Origins 
Theories about the Titans, their origins, and their purpose.

Building on the below Wmg that built on the WMG below it, Titans are weapons made by humans and it was in a war, but instead of something malfunctioning or it being sabotaged the humans with the titans did win the war.
Like is stated below the titans were organic weapons made in a war. Though this war was won, and what we are viewing is a futuristic version of reforming criminals. The original people inside the Walls were criminals that had dangerous personalities and so the government wiped their memories and implanted fake memories that the titans had chased them into the walls. All of these people, or their ancestors were convicted killers and so are all on Death Row, Deadman Wonderland style, where they can be killed at any moment, the mindless titans are the prisoners turned into titans, if your personality is seen to have changed they are required to remove them, so Grisha Jaeger was removed from the community even though he didn't want to because he was seen as reformed. the titans shifters are humans who broke in and are using modified titan technology to fight the system because they see it as wrong to hold humans in captivity like that. When Dr. Jaeger was told about this he escaped and got to the refuge camp, once there he was approached by one or more of the titan shifters and asked to be part of their resistance, he was given time to think about it and a vial of the titan technology, he injected it into his son as a last resort when the outside forces came for him. The coordinate power is knowing without specifically knowing it that the titans were made by humans and if told by someone who is perceived to have authority they are to obey, Eren knows this somewhat, like the same way he can turn into a titan by injuring himself, because Dr. Jaeger told him this when it happened and he still can't remember it all. The Beast titan is the Warden in his own personal titan suit and has no respect for the prisoners, even turning some of them into titans, like Connie's mom. He didn't expect them to speak so he thought about that, and he also didn't expect them to have created weapons to fight the titans, the 3DMG, it's why he didn't recognize it when he saw it. Ymir was one of the titan shifters who invaded the Walls, but she was captured and exposed to the Titan changing process, some leftover memory of her comrades brought her to Berwick, Bertoldt, and Reiner, but her titan instincts took over and she ate Berwick, this gave her the power again and she was able to turn human but she didn't have any of her memories (Why they didn't recognize her i have know idea, I'm hand waving/lamp shading it here, someone edit this and put in a reasonable idea of why they didn't recognize her). The reason of the whole fake king plot was due to the prison staff wanting to keep the people in the prison calm and orderly, so they removed the current king and put in a fake one, and since they were techniquely not allowed to kill any prisoners the staff just let Krista live. All the different people from different races and nationalities was due to this being an international prison, for any murderous, and insane people. This explains Eren's complete and utter insanity being accepted in the world, because just most people were like that, and it explains how Miksasa and Eren were able to kill a bunch of adults with no hesitation or remorse, because it's in their fucking blood. The titan shifters are looking for the coordinate and the Beast Titan, people with the knowledge of what titans are, and the warden of the prison. The only thing this can't explain are why their are only children titan shifters (maybe their are a lot of prisons and this is just a less important one so they sent less experienced soldiers) and why Yuno. Fuck ing. Gas [[Yandere ai.]] Isn't in the prison.

Building on the below WMG, Titans were specifically made an organic alternative to Humongous Mecha suits by an advanced civilization due to resource shortages due to a war. That, and it was just better logistically.
Titans are apparently powered by sunlight, are extremely light and possess no reproductive or digestive organs. All of that is odd for a living creature, but is perfectly understandable if they were created as weapons. The how is unclear but the why could simply be that there were no more resources left. A mechanized suit requires fuel of some kind which would be problematic if you are facing severe shortages due to wartime. However if you can create a means of attack that runs solely on the sun for power, it alleviates that problem. Again, it also deals with the shortage of resources by taking care of any biological needs of soldiers piloting the weapon, as they don't need to eat, drink or sleep, and are even protected from aging, apparently while the "suit" is active. Strategically speaking, enemies can see a huge mechanical battle-suit coming at them from miles away...but a soldier who can form a giant body around themselves from nothing after sneaking amongst you is far deadlier, as the Survey Corps found out the hard way. Somewhere along the line this program was either sabotaged, perhaps by the enemy, perhaps in protest or maybe someone simply made a mistake. Either way, the malfunction caused the pilots to be entirely absorbed into the "suit" until their central nervous system was all that remained, their brains either insensible or lost in a Lotus-Eater Machine while the suits ran on autopilot, attacking anything human.

The Titans are scientific experiments on biological weapons of mass destruction.
This is similar to how zombies are often used in movies, and it would explain how the Titans have so easily taken over as well as how Eren can transform into a Titan. This ties into how Eren's father could be involved in the plot overall.

The Abnormal Titans have the same origin.
This isn't necessarily surprising, since this has already been speculated in-story, but it would make sense if the formula that Eren's father used to turn Eren into a "Human able to don the flesh of a Titan", was the same formula that created the others, only from a different group of humans.
  • At least partially confirmed with the revelation of who the other Abnormal Titans aside from Eren are. Whether they were given the same formula or not has yet to be revealed.
That sounds weird, but what else would be the reason that they continue to thrive, despite the fact that Titans don't have any sexual organs? The other Titans running around might be an imperfect process/procedure of what Eren and all the other Titan Shifter's might possess.

The Titans are the result of a disease or genetic mutation, artificially created to control the population.
Eren's father was working on finding a cure when the Colossal Titan attacked. He was planning to show Eren what was in the basement because he knew it was almost ready.
  • Eren's father was getting too close to a cure so the Armored and Colossal Titans were dispatched to destroy his town.
  • The tribes/clans that the Titan Shifters are from are part of a shadow organization that wish to maintain the status quo, which may involve the king. They use the people's fear of Titans to stay in power, therefore knowledge of the outside world is forbidden.
  • Top recruits are pointed at the Military Police, so the Recon Corps is devoid of exceptional talent. The organization then sabotages the efforts of the Recon Corps so that inquisitive and capable leaders die in the line of duty and humankind neither gains nor loses ground.

The Titans were made as an effort to unite humanity, and it backfired horribly.
At one point, Pixis talks about how, once upon a time, humans squabbled over race and principles, and "someone" once said that if humanity had an enemy that wasn't human, they would have to band together and put aside their differences to succeed; Eren answers that he's familiar with that piece of folklore. It's more than just folklore; it's the explanation for why the Titans exist, warped by generations' worth of word-of-mouth into ordinary apocrypha. It may also explain the significance of the Walls: they represent strength through unity. Especially if the first Colossal Titans volunteered to serve as the foundation for the Walls, hoping that they could further humanity's goals by doing so; it fits in nicely with the conceit of self-sacrifice, because if the Titans hadn't been made at all, none of the atrocities in the story could have happened.
  • Although the author confirmed the Walls do not symbolize strength through unity — the theme behind the Walls is the complacency in humanity — as opposed to Eren's adventurous desire to see the outside world.
    • I meant more in-universe than an actual symbolic motif; ie, it's what the Wall Cult's religion is based on.
  • The Genius Bonus in the "runes" might support this: by the time the Titans came around, humanity had already come close to wiping itself out.

There are multiple ways of creating Titans out of humans, but a specific way of creating Titan Shifters.
Castle Litgard/Utgard for some reason showed recent signs of human habitation despite being close to the Wall. Considering that the lettering on the can of herring that Ymir found was of a different language and the Sasquatch Titan was concerned about language barriers, the food that Ymir found is likely to be a source from outside the Walls and contains a trigger to turn humans into Titans. Also, the recent inhabitants were from beyond the Wall.
  • In the case of Eren, it seems like to become a Titan Shifter, one must have to have the transforming serum injected directly, which is why when ingested as food, it only turns you into a normal Titan, not a Titan Shifter.

Titans are the inadvertent results of research into immortality.
May have some evidence, with Ymir revealing that while transformed they do not age. She is over 70 years old.

The purpose of the Titans
Based on these two posts on tumblr, there might be some additional info on why/how the Titans were created. It does seem to be just a draft/prototype, but who knows, some of it may still apply to the manga in its current form.

So, considering the information in here, the Titans were artificially created, in this case by some sort of cult (probably the progenitors of the Wall Cult) who was worried about the environment. The purpose of the Titans was to exterminate humanity and protect the environment, which is why the Titans ignore plants and animals. And, it seems that the Titans 'absorb' people, unless they have a 'pure heart', in which case they can become Titan Shifters. However, regular people without that pure heart are instead turned into Titans? This may be what happened to Connie's village
  • Admittedly, it makes sense to assume this has changed at least a little, given that while one could probably make an argument for the "pure-heartedness" of Eren (a pure heart for wiping out predators!) and Reiner (the pure heart of a big bro!), Annie thinks those with pure intentions are rare exceptions to the general conniving selfishness of humanity and explicitly identifies herself as being of the latter type, while Bertholdt's motivations remain opaque but merciless. And, most blatantly of all in the last few chapters, Ymir is pretty categorically not pure-hearted by any definition.

Titans WERE created to unite humanity...
... but not by giving them a common enemy, as the legends suggest. Rather, Titans were created in an effort to remove all sources of human conflict and thus end war forever.

Remember that the Titans first appeared after humanity had already crippled itself by terrible wars, which decimated the population. Now consider the Titans. They need no food, no shelter, and no material goods or tools to survive. They don't age and as they can regenerate most wounds, we can assume they are immune to disease. They have no means of natural reproduction (that we know of), and they don't attack each other. And finally, they are, as far as we know, mindless, and have no society.

Each of those traits is a direct counter to a common cause for human warfare. With no need for food or material goods, there is no need for war over lands to be used for raising crops or gaining valuable resources. With no need for reproduction, there is no need to fight over family rights and breeding ability (though that is admittedly more a thing that animals fight over, but humans can do it too). With no minds, they have no ideas, and thus can't fight each other over differing ideologies. Finally, with no social stuctures, there is no chance of war between different classes or between the oppressed and oppressors.

As it is mostly accepted that the Titans are transformed humans, we can assume that whoever created them made them as a means to transform the human race into a population that was above warfare. Titans are essentially immortal, and at the same time, their "pilots" are either unconscious, or dreaming. They were created as Lotus Eater Machines to simultaneously keep the humans alive (whoever created them was a Well-Intentioned Extremist who still cared for the human race, in a way) and stop them from warring. The Titans' only real flaw, the fact that they attack other humans, is simply the means of tying up loose ends. The Titans seek to destroy all the humans who remain humans: that is, those who are still capable of committing warfare. Once all of these who refuse to give up warfare are dead, the Titans will just sit around in peace, and humanity will be "preserved" without war. It's an Assimilation Plot writ large (and bloodily).

As for where the Titan Shifters fit into this, either they are abnormalities that whoever started things didn't foresee, or they were created by a different process which other humans created separately in order to oppose the Assimilation Plot.

  • So the Titans are basically Reapers made out of flesh. Yipes.

Titans were created to unify humanity but the plan was sabotaged
The Titans were an attempt at solving two big problems at the same time. First, they were/are an attempt to unify humanity peacefully. As stated in another WMG above, they have no need for food, no ability to reproduce, are nigh immortal, etc... Except in this case, they were always intended to keep their human sentience. The idea being that once the more basic needs and causes of conflict were removed from humanity the remaining ideological differences would be easier to solve.

Also at the same time with the Titans were developed for interplanetary/interstellar exploration, as being solar powered they make good exo-planet exploration suits. As they need nothing but sunlight they have no real limitations on where they can go. I'd bet the Titans wouldn't even need to breath beyond the need to speak. Why send a rover unit only capable of mechanical analysis when one could send the scientist himself, capable of thought and making decisions on the ground? That was actually the first stage of the Titan shift program. A group of astronauts and scientists were shipped off to explore a likely habitable exo-planet (or possibly one of Jupiter's moons) and test the hardiness of the Titan transformation.

So the humans of this time had the advanced technology to make the Titan bodies and at first they would be tested as a method of exploring other planets. This was successful and the early explorers being able to setup a colony on a more habitable planet (this being the origin of the Walled City.) Then once the technology was proved safe the general populace would be made into Shifters. However, before the Titan program was implemented among the general populace, it was sabotaged by a dissenting group of some sort who made a few changes. After the sabotage, rather than everyone maintaining self-consciousness when the Titan change was implemented only those with extreme will power or an immunity (natural or acquired) to the change made in the Titan shift could maintain their self-consciousness when shifted. Secondly as well as the loss of high consciousness there was programming put in that forces the mindless Titans to attack and kill those humans as of yet unchanged (or Titans unaffected by the sabotage) so as to prevent this sabotage from being undone and or to keep the chaos mounting to allow this extremist group more time to enact further plans. This modification explains the differences between the Shifters, the aberrants and normal Titans. Shifters are clearly those humans with an immunity to whatever change was put in the Titan program. Aberrants would be those people who had enough willpower/immunity to maintain partial control of themselves but not full consciousness, thus why their attack patterns are less predictable. But still so far gone that the aggressive human killing programming and general loss of consciousness to have effect. Whereas normal Titans are simply the majority of humans without the ability to resist the changes wrought in what was to be one of humanity's crowning achievements in bio-engineering. Though as seen it is possible with time for a person to gain the willpower needed to change from a normal Titan to a Shifter.

So it was only after the Titan program was implemented on a wide scale that the changes were noticed. Humanity did its best to control the beasts they had made of themselves and when that failed attempted to eradicate those Titans that they could in an attempt to save those who had witnessed the changes and wisely decided to not titan shift until it could be fixed. Unfortunately, they were sabotaged again and this time were forced to leave Earth in favor of the colony that had managed to place. But the saboteur group that had made the initial changes to the Titan program had moles in place, too (the precursor to the Wall Cult). When they landed, the colonists decided that they would abandon Earth for the time being until they could fix the problems with the Titan change (the resetting of the calendar 850 years ago) as it would be a fairly simple matter to fix those already transformed once they had solved it. As the colony was started a large portion of those moles embedded in the colonist were discovered and either fled or were kicked out by the rest of the colonist (explaining our Shifters from beyond the Walls) with only a few of cleverest being able to maintain their cover. Those where were then cut off from the main group evolved into the Wall Cult on the inside, unable to maintain contact with their exiled kin until long after the colonies were founded. The reason this is unknown to present day inhabitants is centuries of cloak and dagger actions bent on hiding and destroying the truth. Humanity is now so far degraded in its scientific and technological prowess that it will be centuries if not millenia before it regains its abilities and can return to Earth which will by then be a long forgotten legend.

Titans are dead people, brought back by a science project and resurrected.
In the past, a great war was mentioned. The higher ups in that war tried to discover a way to bring their lost soldiers back to life and fight for them again. They succeeded but the result was the Titans, mindless creatures that didn┤t saw the difference between friend or foe. To make sure that a injection would not be necessary the Titan shifting was turned into a added formula that spreads through the Titan's saliva and bites. They turned to their creators and forced mankind into the state we see them as now. Titan Shifters are a result where the dead person finally is fully resurrected and reverted to its human form. Doctor Jeager re-discovered the original forumla, tweaked it and injected it into his son as a last desperate effort. His basement contains the research notes and true history of the war. Also notes of how to produce the formula which he perfected in his occupation as a doctor. He entrusts Eren to give Humanity a chance to fight back against the flawed creations and the Higher ups that chose to Titanify and regained their senses that way. Beast Titan has the formula and is using it to create decoys for its true objective. He is an outsider who invades like a general of war, sacrificing his troops without any concern or remorse. He might be one of the original Shifters from a time before the Walls and have knowledge of the war. The BAR is working for the opposite side of the war, their higher ups are just as cruel as the Beast Titan. That┤s why they are afraid of failing. All while the mindless Titans continue their war-programmed mission. "Kill all humans." Like a machine without any regard of what they are doing is right or wrong.

The Titans were NOT created by humanity.
At least, not entirely. Why? A couple reasons. A: It's just waaay too obvious. Sure, Annie being Titan Chick was obvious too, but this is Myth Arc -level stuff we're talking about here. B: People are constantly talking about how Titans are humanity's "natural predators", and the narrative frequently seems to almost say that they're inherently superior to humans. None of that makes sense if they're just a failed (or worse, successful) experiment. B: They're just wrong. They feel so alien, particularly the furry guy. Something about their origin just pretty much has to be organic and otherworldly in nature, rather than just being some accident.

In any case, this business of unity in the face of a common foe means that to some extent, humanity (or, actually, perhaps some alien force working toward human unity...actually that makes a whole lot of sense...okay I'm continuing this down in the next one after I finish this WMG) probably was responsible for the current state of the world, perhaps causing the humanoid Titans, while the apes were the originals.

The Titans were created to unite humanity, but not by humans.
At least not the humans in the series. Given the confusing nature of the whole "in 2000 years" thing when it doesn't appear to be a post-apocalyptic Earth, it seems possible that the people behind the Titans are in fact future humans from Earth, who created the Titans in an effort to force the presumably stagnant society to evolve. Eren and company, meanwhile, are either Human Aliens or the descendants of colonists or something. Of course this means that Future Us are either well-intentioned extremists or the Titans don't actually kill anyone, but rather teleport people to Earth or something and replace them with dummies or holograms that are then messily torn apart.

The Titan / Human livestock metaphor is backwards.
The Titans were created out of humans to be the perfect livestock — the require no food, are immortal, and any meat you slice off of them grows back. They are the perfect food and the solution to humanity's hunger. The reason for the Titan motivations is basically payback — the people turned into giant buffets either consciously or subconsciously fought back against being turned into giant cows and fight back by attempting to eat humans in a bit of irony. Knowledge that this is how the world was "supposed to be" is also the motivation among the Titan Shifters attempting to end humanity.

The Titans are an Environmental Response to Humans
When you look at them, the Titans seem perfect for the purpose of protecting ature from humans. They don't die, they don't devour other animals or plant life, they can't accidentally overpopulate and once the last human was gone they wouldn't do any damage to anything else.

The coordinate was used to make the mindless titans into man-eaters. It was stolen from the shifters home town
The titans eat humans out of impulse rather then a desire to. The titan that killed Ilse looked like it was trying to restrain its self from eating her. I believe that when the titans were created they didn't have this impulse. Like when Eren overuses his transformation, they were always fused to the bodies and would become violent but didn't eat people. When the impulse is not presented (either not added or there are no humans near by) they act more reasonable, such as the talking titan calming down after eating Ilse. The people who hadn't been been transform recreated the process that heals the titan's body and applied it to humans. They used the coordinate to make the titans eat those people to regain their human bodies and become "warriors".

However humanity was fighting along itself. A portion stole the coordinate and granted it to two people. One created the walls and modified the memories of the people who went inside the walls. The others is the beast titan who made the titans all into man-eaters to make them become a single common enemy for humanity. The remaining people who created the coordinate was almost completely destroyed by the beast titan. They sent out bra to reclaim the coordinate by letting the titans into the wall. If the titans eat the person with the coordinate they would either see an intelligent titan like the beast or a titan shifter. When they saw Eren they thought he was outside and had regain humanity inside the wall. Once they found him they no longer needed to destroy the walls and changed the plan to capturing him.

     Theories about the Titan Shifters 
Theories concerning Reiner, Bertolt, Annie, and Ymir.

Ymir's "endless nightmare" has less to do with being a mindless Titan and everything to do with her past
From what Ymir told Bertholdt, it can be surmised that her dreamlike state was very similar to Eren's in Trost. His hallucination was anything but a nightmare; in fact it was a peaceful vision of being surrounded by those he loved, which makes sense. Eren's past was for the most part peaceful; he had a stable family who cared about him, he lived in a peaceful area, and his family was financially stable. From the few mentions of her past, Ymir's life was probably the exact opposite. At the very least, she was hated just for being born. It could be that her nightmare simply consisted of reliving her ugly past, unable to ever escape.

Theories on the Titan's motivations.
Yes, this includes the human Titans. Why do the giants feed off of humans when they clearly don't need to in order to survive? Why wipe out all humans, who've been living rather peacefully within the Walls? Because they were picking off weaker Titans outside of the Walls? It's been made clear that the human Titans see them as nothing more than tools for their goals to wipe out humanity and get Eren back.
  • The Titan Shifter trio (Reiner, Bertholdt, and Annie) are Well Intentioned Extremists, or at least started out as such. In particular, Reiner's Villainous BSOD and Annie's admiration of people with strong convictions both seem to suggest that they genuinely believe their actions are for the greater good and justify the horrible things that they have done. They also don't seem to be operating from a hatred of humans, or any other sort of malice and seem relieved when the discovery of Eren means they no longer have to continue the extermination. Beyond that, their most obvious goal of smashing a hole in each Wall could have been accomplished without joining the military. As refugees, they would have still been able to reach the Walls, smash them, and then blend back into the fleeing crowds of civilians. But instead, they spent two years apparently just blending in before joining the military, even though all three were already eligible for recruitment. They waited two years, and then spent another three in training, before making their move again. And all three had originally planned to join the Military Police, suggesting a goal beyond simple destruction of the Walls.
  • Another theory concerning the Trio and their motivations. Their village/people are a Proud Warrior Race, which explains all their discussion of themselves as warriors (as opposed to the human soldiers). Annie and Reiner both criticize themselves for failing as warriors, while Bertholdt passive aggressively states Reiner used to be a warrior after he risks his life to protect Conny. Considering the massive amount of Double Meaning going on with them, it's possible that when Reiner and Bertholdt discuss their motivations for enlisting.....they're actually talking about the ideals of their own culture, not the one within the Walls. Reiner focuses on ideals of duty and personal effort, while Bertholdt focuses entirely on what society expects from the young.
  • There's another society comprised of Titan Shifters, and they're at war with the human settlement we see in the story.
    • But why would snagging Eren be enough for them to leave humanity in peace?
    • Two theories. A) The Titan Shifters feel it's better to get the super-weapon out of humanity's hands than just wiping out humanity. It's like leaving a warring faction so humiliated and disarmed that they can not fight. Second theory: They changed their goal — seeing Eren as an alternative to wiping out humanity. To them, using Eren is the more expedient of the two options.
  • Going off the below guess that the Walls were meant to crumble and release a wave of giant Titans to devour all of humanity, the Shifters' goal is to stop that. They knock holes in each wall and let normal Titans in to cull off humanity's population, with the intent of keeping them from reaching numbers such that the Wall Titans will awake to feed on them. After all, if such massive Titans were released, the Shifters' own homeland would be in danger. However, Reiner and Bertolt believe they can bail that mission if they bring back Eren, Krista, or the Beast Titan. Those are the man suspected, and later confirmed, to have the Coordinate, the heir to all the Wall Cult's secrets, and an intelligent Titan who clearly dislikes humanity and has some ability to command Titans, although it seems it is less powerful or effective than the Coordinate.

On "Ymir-Sama"'s origins and the Talking Titan
Building on the theory that the Walled City was built as a compromise between the humans and the mysterious leaders of the Titans and that there have been multiple mass human sacrifices in the past, would explain some of their relation. Right before she turns into the Dancing Titan Ymir says/thinks that she "died for the happiness of a great number of people but was given a second chance" and that she was "hated simply because she was born". A large part of her attraction to Krista/ Historia is due to that shared trait. Ymir could have been the illegitimate daughter of a noble family offered up as part of a mass sacrifice to the Titans (or not even a noble family; an illegitimate child would be a shame for a commoner just as much as a aristocrat). By some lucky chance, she managed to steal and inject herself with whatever creates Titans. Unfortunately she bit off more than she could chew as she spent sixty years as a mindless Titan. If she is from a noble family, it could be that she had a loyal retainer or butler of some kind who volunteered to go with her to her death and she took the chance to "save" him as well by doing the only thing she could, injecting him with the same serum, making both of them mindless Titans. It would also explain why the Titan was tearing at its face; it was trying to resist the urge to eat what it thought was Ymir, thus its stricken expression as it chews on Illse. On some level it is aware that it is eating the person it was supposed to serve/take care of.

Speaking of Ymir...
There is one character named Ymir, and she turns out to be a more benevolent Titan Shifter. Is it a coincidence or is she the leader of some Titan-turned-human race or something?
  • Considering the author's use of Meaningful Names (Jaeger, Historia), I'm pretty sure that he's giving us a clue that Ymir is a leader of some sort... In Norse Mythology, Ymir was said to have been the ancestor of all Titans.
    • Some bonus pages in volume 12 have Ymir say that "Being a goddess isn't such a bad feeling."
      • Not necessarily in support of ymir=goddess/benevolent titan shifter idea- but Ymir may be older then the 60-70ish age people have concluded. Things to consider:
      • 1. she knows how to read a dead language/different language then whatever is the common tongue. That can of herring that reiner couldn't read could be +100 years for all we know.
      • 2. She doesn't have any titan training- meaning the in the time she was turned into a Titan, the idea of such a thing has not yet reached her or did not even exist.
      • 3. Looking at connie's mom who got turned into a titan by the beast titan, we can conclude when humans are turned into Titans, they retain some of their original appearance (especially facially). Assuming that Ymir was 'turned' into one through some similar means, how old was she when she was turned into her 'dancing titan' form? Where as Annie, Reiner and Eren all have appearance semi-congruent with their human body 'age'. Ymir and Bertholdt do not- particularly, her titan form always reminded me of middle-aged witch-(Bert's being skinless and hairless is also difficult to judge age-wise).

Reiner will end up either undergoing a Heel-Face Turn or Redemption Equals Death
When asked about what Reiner was like in the past, Bertholt is critical of how he's no longer a warrior. In fact, Bertholt makes suspicious or annoyed looks whenever he sees Reiner acting close to any of the humans. This is because Reiner is Becoming the Mask, and he's already admitted that his friendship with the others has left him deeply confused over his mission.
  • Related: Reiner sabotaged the attack on Wall Rose. Whether he did it consciously or not, he's pretty clearly hesitating to do his part and quite literally jumps at the first opportunity to get out of it. Mikasa and Jean leading the charge towards Headquarters simply gave him an easy out, with Annie and Bertholdt both visibly shocked when he joins the other trainees.
    • And now looking very uncertain, with the revelation that his behavior is the result of having gone mad from guilt. He wasn't sabotaging their plans, or doubting his mission....he quite literally had moments where he thought he was human and didn't remember his true identity and mission. Based on him being driven that far over the edge because of guilt, Redemption Equals Death is still a possible outcome.
    • Reiner will undergo a Heel Face Turn. Bertholt will not. The conflict will be milked for as much existential and emotional drama as realistically possible. Given Reiner's status as Ambiguously Bi, it's possible that this will end in Love Redeems for Bertholt.
    • At some point, Reiner is going to have to face himself and decide if he's a "Soldier" or "Warrior". However, given what his mental state was like when the big secret came out, he may not have much time left before he completely snaps. And if that happens, well...
    • Not to say that there isn't hope for him either. Spending some time away from the others might give him a chance to get his head on strait. Considering that the Titians as a whole have been revealed to be Tragic Monsters as well as a government conspiracy possibly connected to their creation, there's a distinct possibility for an Enemy Mine situation.

One or more of the Titan shifters in general will undergo a Redemption Equals Death
Given how all of them show have shown various levels of guilt, at least one of them will end up switching sides and do their best in assisting Eren and co, though at the cost that they end up killed. Said outcome will also be due to You Have Failed Me by whoever their boss is.

Ymir is Titan royalty, or what passes for it.
This one's probably fairly obvious, but Ymir seems to have a pretty good grasp of Christa's situation, and she also seems to think they have a lot in common... but they really couldn't be more different, except for their one conversation about their names.
  • The talking Titan from Ilse's notebook mistakes Ilse for Ymir.

How Titans Shifters form Titans out of nowhere
  • They use the elements from their surroundings, or they use the Digimon Tamers Realization(Bio-emergance) theory. False proteins are used to build up their Titan forms minus the blue cards and the portal from another world.

Annie, Reiner, and Bertolt are not the only agents living within the Walls.
Regardless of how powerful they are, it seems unwise to leave three children to their own devices for five years. Reiner also sounds pretty confident that Eren agreeing to come with them will end the attacks, something he probably wouldn't be able to decide completely on his own. Beyond that, Annie was living in the Inner Districts as a member of the Military Police, while the other two were living near the edge of civilization with the Scouting Legion. Coordinating their plans would be much easier if they had others to serve as messengers between them, never-mind that their orders changing would require them to have someone with authority acting as their handler.

Annie was not the one responsible for Marco's death.
Bertholdt was the one that actually did it, and Annie was merely the one that ended up using his stolen gear. Annie is shown to have hesitated to almost dangerous degree when it came to attacking her former comrades from the 104th, and is visibly bothered when asked how she ended up with Marco's equipment. Reiner also ended up helping the 104th during the Battle of Trost, shooting their whole mission in the proverbial foot in the process. On the other hand, the most recent chapter shows that Bertholdt has a complete Lack of Empathy when it comes to anyone outside his own group. Not even Eren gets an ounce of sympathy from him, and he was in the end the only one to fully carry out his mission without fail. As such, he seems to be the one most capable of killing one of their classmates in cold blood if necessary. Annie was simply the one assigned to carry out the destruction of the two test subjects, possibly because she wouldn't stand out as much as super-tall Bertholdt or built-like-a-linebacker Reiner. Her build is perfect for blending in, and wouldn't allow them to have any sort of defining traits to look for when searching for the culprit.
  • This theory is definitely plausible, but Bertoldt does show himself to be empathetic and remorseful over his actions in Issue 49. However, he is still obviously the one most willing to carry out his task.

Reiner's dissociative episodes are the result of the Battle of Trost.
During the battle, Annie and Bertholdt both follow his lead. Annie is an established lone wolf, and probably wouldn't have asked him what their next move would be, if she had even suspected him of being psychologically compromised. But we see very clearly that the battle, and the cleanup afterwards deeply effected both Annie and Reiner. For the first time, he was forced to truly confront what their actions would cause, as they watched their classmates being Eaten Alive and later had to gather up the remains for disposal. Such an experience would be traumatizing enough, as shown by Jean's Character Development. But Reiner had the extra push of being partially responsible for all that suffering and death, to people he actually knew. Annie leaves within the next few days to join the Military Police, and continues to trust him to handle providing her with information and assistance. Again, I cannot see her relying on him if she'd suspected him of having bouts of crazy. Likewise, in the aftermath of their first battle, people probably wouldn't even notice if Reiner started behaving strangely or was having problems with his best friend. Anything unusual would be written off as him simply being a Shell-Shocked Veteran, as opposed to a villain being crushed by guilt.

Sexuality is meaningless among Titan Shifters, and Reiner isn't bisexual at all.
Sex doesn't have any biological meaning to a race of creatures that can't breed anyway. They're clearly capable of love and caring and possibly even of sexual desire, but culturally and socially, sexual orientation isn't any more significant than hair or eye color (maybe even less so, if Ymir's dark hair and freckles are any indication). Reiner isn't bisexual: as a Titan Shifter, he's gay, but his human other-consciousness is straight, because that's more typical of humans. His crush on Christa is his mind's way of reconciling the conflict between his loyalty and belief in humanity and his mission as a Titan Shifter spy.
  • To be fair, there's no evidence that their people can't reproduce sexually and we don't know whether they all have powers or not. I mean, Annie had a father and nothing ever stood out about any of them during their three years of living with very little (if any) privacy. But in terms of culture, it's quite possible they have different attitudes concerning sexuality.

Annie and Reiner are artificially-made Titans; Ymir and Bertolt are not.
Bertholdt doesn't seem to be unique in his Titan form, if all the other Titans in the Walls are also Colossal Titans like he is, and Ymir is apparently a member of her own Titan shifting people, but Annie and Reiner are fairly singular: for one thing, they both have strongly gendered appearances, they're both blonde, and generally resemble their Titan forms much more than the other two do. It may also explain Bertholdt's coldness where Annie and Reiner show some care for their fellow soldiers; going by Ymir's conversation with Christa about not being what others want, she may be rejecting the same approach Bertholdt is taking on principle.
  • It should also be noted their appearances and abilities are closer to Eren's than any other Titans, and we know that Eren's powers are artificial in origin. Considering how they function, it's possible Reiner's powers were tailored so that he could serve as a bodyguard and partner to Bertolt.
    • Jossed, at least for Ymir. She stole the Titan's Power, and only regained control of her powers 5 years ago.
      • In fact it is proved for Ymir and Bertholdt. She was a normal Titan and then she became a Shifter and, according to her dialog with Bertholdt, so did he. But we know neither abour Reiner's nor Annie's past before they became Shifters. However, I suppose all the Titans are artificial, but ERA are simply a second, improved generation.

The village the Titan Shifters come from is a village populated...
...by former Titans. Chapter 47 shows that Titan Shifters happen when a Titan comes out of the dreamlike state and becomes human again. It's happened more than just with the Shifters we've seen, and so they have a refugee village out there somewhere.

The story Bertolt tells Eren and Armin is mostly true.
He merely lied about the when and where. And the reason that he states he doesn't remember what happened after seeing the Titan at his window is because he was telling them about the day his people were turned into Titans. The reason Reiner and Bertolt immediately recognize the Ape Titan is because he was there that day, and had a hand in their transformation.

Reiner lied when he claimed that exterminating humanity was their original goal.
The destruction of Wall Maria and the attack on Trost were merely a means to an end, not the actual goal. The original attack was merely a diversion, to allow the trio to infiltrate without trouble. It may have also been done in an effort to bring the Beast Titan out of hiding or awaken him, accomplishing one portion of their mission. Since they seem to believe the "Coordinate" is a person, or a person will help them find it, exterminating humanity completely would be detrimental to their goal since they risked killing the very person they were looking for in the first place. Of course, there's always the possibility that the trio were unaware of the full nature of their superiors' plan, considering Ymir calls them "small fries" and suggests killing them won't accomplish a thing.
  • The Titan Shifters may be just as in the dark as the Survey Corps.

Why the Titan Shifters are also targeting normal humans
Given the practically-confirmed idea that normal humans can be transformed into mindless Titans, the goal of the Titan Shifters is to wipe out the source from whatever direction possible. Titan Shifters can be assumed to be immune to whatever the apelike Titan does to transform other humans into Titans, and they are still targeted by the mindless Titans. Somewhere there's a settlement consisting entirely of Titan Shifters, and their ultimate goal is to take out either the Beast Titan, the humans he's capable of transforming into mindless weapons, or both.

Alternate reasons the Colossal Titan let the mindless Titans in...
  • Titans must consume a certain amount of humans before they're capable of regaining their own humanity as the assumed-to-be-natural Titan Shifters did. The goal of the Titan Shifters is to take out the apelike Titan so he can't make more Titans, and then make sure the Titans already existing have a chance of regaining who they were.
  • To lure out the Coordinate. Logically, if they were looking for someone in hiding who could control mindless Titans, the fastest way to expose that person would be to actually threaten their home with a whole bunch of them.

The Titan Shifters are out to destroy current humanity in order to protect the future of humanity that Titan-kind represents.
As long as the Walled City exists in safety, there will always be humans to continue fighting the Titans; it's an extremely slow battle, but if Titans don't reproduce on their own until after they've changed back, and changing back could take centuries, they're eventually going to whittle the population down. People like Levi and Mikasa might only come around a few times in a generation, but that's still tens of Titan losses each, and those numbers don't get replenished. Since humanity has proven that it'll never completely give up the fight, wiping them out is the only way to ensure that the Titans will live long enough to become themselves again. Of course, the Titan Shifters would focus on saving the potentially billions of future Shifters, at the cost of comparatively few ordinary humans.

It might also explain why the Walled City exists: to give the remaining humans a chance to keep interbreeding, to raise the chances of any Titan being able to eat any human and be able to change back.

On top of that, it might also give us an insight to who "Ymir's People" are. Maybe they're like the "universal donors" of the Titan world; eating one of them could turn any Titan back to normal, and finding out that Ilse wasn't one of them made that one particular Titan so disappointed, it lost its grip on what little sanity it had managed to grasp, inspiring it to tear into its own face and devour her anyway.

Annie's father told her to hate the world before she ate him.
Adding to the above: it's a cruel world indeed that would make a child eat their parent in order to become strong enough to change it.
  • Episode 25 shows some huge evidence for this. Annie's father tells her that even if she becomes the whole world's enemy, he will always be on her side... and that she "must come back", and he says all this while she's completely cold-shocked and he's all but in tears exactly like Grisha was when telling Eren that he has to make it back to the basement. Those were Annie's father's last words to her.
  • Annie's Visual Novel seems to contradict this theory. Annie thinks of him in present tense, with the goal of seeing him again. His words are also explained to have been a coded message to her, asking her to abandon her mission and return home safely rather than risk her life.

Bertolt has the greatest limitations on his Shifting abilities.
While Eren's inexperience results in him being in a weakened state after transforming, he's still able to do it more than once per day. The only Shifter thus far to not be shown transforming more than once in a single day? Bertolt, the Colossal Titan. Even when he and Reiner are very possibly about to die, he makes no efforts to transform and instead relies upon Reiner's protection and his damaged 3D Maneuver Gear. More than likely, the Colossal Titan is such an extreme transformation that he has very limited ability to use it. When wounded and exhausted, he's only shown performing a partial transformation and he's never been shown to maintain it for more than a few minutes at a time. As such, it's possible that while he retains the stamina necessary to function after changing back, unlike the other Shifters he requires a much longer cool down period between transformations than any other Shifter.

The Titan Shifter faction that Annie Bertolt and Reiner are a part of is in really bad shape.
This is more me going off on a gut feeling, but considering how important it seemed to be that they find the Beast Titan and Eren, I find it odd that such important tasks were given to teenagers. Them being in really bad shape is the only explanation I can come up with.
  • Well, being teens also helps them to get close to Eren easily. Putting all 3 of them in one unit was a bad idea though.

The Titan Shifters are Well-Intentioned Extremists and/or Good All Along.
Yup. The reason they wanted the "coordinate" in the first place is to control the Titans and divert them away from humanity.
  • It is becoming apparent that there are more threats besides the Shifters, both outside and inside the Walls.

Annie, Reiner and Bertolt are not their real names and they have Norse Giant names like Ymir
Bertolt could be Bolthorn (frost giant and grandfather of Odin in Norse mythology) for example.
  • Unlikely, as they never refer to each other by different names even in private or when doing so would be useful — such as Bertolt snapping Reiner out of his delusional "Soldier" state.

Titans and Titan Shifters are the curse/reward of the God(s) of their world.
Using the anime for reference, every Titan-shift is represented by a Shazam! styled lightning bolt, all followed by one common element — a blood sacrifice. Taking it literally, it would mean that the person who can Titan shift would need to give up blood in order to transform, but let's take a look at this a little deeper.

Eren, being the protagonist, is the face of the whole theme of the story: stop being complacent. Although his Titan shifting abilities *may* be artificial, the underlying principle is the same — he is willing to pay, to give up something, to use the Titans' power. The problem he had when he tried to transform against Annie the second time was that he refused to accept that Annie was the enemy — he refused to give her up, thus he could not 'pay' the price to the gods.

The blood sacrifice is symbolic in the fact that the Titan Shifters are willing to do whatever it takes to accomplish their goals.

Ymir claimed she stole the Titans' power, she took it without paying for it. She had not 'earned' it, thus the mindless Titan she was for at least 60 years. Ymir thought she was being clever, yet whoever was watching over the world caught onto this and, like all humans who are not willing to pay for something, Ymir was punished for her hubris. During her endless nightmare, she would eventually find a way to prove she was finally worthy — finally willing to give up everything just to get what she wanted. Thus, the curse was lifted and she was given the blessing.

Annie, Reiner and Bertolt are all completely in the dark about the Wall Cult and the Military Police's attempts to sabotage the Survey Corps
Something is just bugging me about this. When Reiner and Bertholdt were talking with Ymir, they pretty much spelled everything out. They talked about the Coordinate, their village, how their mission was to wipe all of Humanity out and how Eren's existence could change that mission. Not once did they ever bring up the Wall Cult or the Military Police. Even Annie, who was part of them, seemed unaware of what her superiors were up to. I mean, if she had known that the Wall Cult was gunning for Eren, wouldn't she had told Reiner and Bertholdt right away? And wouldn't the two of them saying that it wasn't safe for Eren inside the Walls BECAUSE of the Wall Cult be an excellent argument for why he should've gone with them? Just like how the Wall Cult is keeping the Survey Corps in the dark about the Titans in the Wall and so many other things, Annie, Reiner and Bertholdt's village could be keeping them in the dark about what's going on inside the Wall.

Annie isn't a Titan who became a Titan Shifter; she is a girl who was selected to BECOME a Titan Shifter.
The same may go for Bertholdt and Reiner, but it explains the similarities between Eren's transformation and her own (strongly gendered appearance, traumatic childhood scene with terrified father pleading as if he isn't going to be there much longer). The people of her hometown chose her to take the same injection (or a similar one) that Eren did, which would cause her to eat her father like he did, for the sake of the plan to infiltrate the Walls. It also makes sense, because Annie could summon normal Titans to come and eat her body. She's a failed Coordinator.

The Titan Shifters had to look for the Coordinate/Coordinator within the Walls because the materials that give the Coordination ability had been stolen from them.
Grisha knew that the coming of the Colossal Titan and the breaking of the Walls to let the Titans in meant that there would be spies, because he knew what Titan Shifters are. He made Eren a Titan Shifter because he needed the Coordination ability to go to someone he knew would use it for the sake of humanity.

Shifters don't have the ability to turn into Titans.
They have the ability to turn into humans, and this is what Ymir stole. More specifically, we've seen three things associated with a Shifter's transformation:
  • The Titan body appears out of nothing in a flash
  • The human body gets very rapidly regenerated to full health (seen at Trost with Eren)
  • While there might be a reverse transformation (we didn't see corpses for the Colossal and Armored Titans), we've never seen anyone use it — instead, they hack out like disposing of any normal Titan corpse
What this potentially implies:
  • The Titan body is literally transformed from or otherwise destroys most of the pilot's human body (which explains why no pilots have ever been found and why Titans other than Eren don't much care about being stabbed in the wrong part of the neck)
  • Shifters can reverse the process simply because they have an advanced regenerative ability for their human parts as well, which lets them survive the process
  • Normal Titans are just a shattered brain and spinal cord slowly cooking from heatstroke acting as a control chip at the back of the Titan body's neck
  • The regenerative ability is a chemical compound that can either be injected (Eren) or eaten (Ymir)
  • Combine regeneration and transformation chemicals, and you get a Shifter
    • The problem with this idea is that the only known way of creating Titans is by turning humans into them, and that the Titan's bodies ignore the laws of physics except where the human "pilot" goes. Both these suggest that a human form is the baseline.
    • Confirmed on all points as of chapter 57.

The Shifters are HeroAntagonists
I have a theory under the Beast Titan sections explaining that he's their enemy and the story's Big Bad. Expanding on that, it appears they're only trying to destroy the Walled City to stop him from wiping out not only the humans within but every human on the planet, and to prevent the creation of more Titans. The reason Reiner is so worried that Eren can control Titans is because he knows Titans are really humans, and Eren's genocidal hatred will cause him to murder innocent people trapped in Titan form. They've also tried to get Historia to come with them, implying that they don't know the origins of the Walls, and need that information to stop the Beast Titan. Annie encouraged Eren to question the status quo because they're aware of the government conspiracy providing the Beast Titan with more humans.
  • A few nitpicks to your theory: The Beast Titan in unaligned with the (current) government, as it does not know anything about their equipment which has been in use for 50+ years and Ymir knew about the Beast Titan even though she'd been out of commission for 60 years. So while it may have had ties to humanity in the beginning of the Wall Society, it hasn't for several generations. Second issue is that Reiner and co. know about the origin of the Walls, as he explicitly mentions "the contents of the Walls" and states they need Historia because her family may know more about the Coordinate. He tells Bertolt they need her in case Eren turns out to be a false lead. We also currently don't know the Shifters' relationship with the Beast Titan, other than the fact they wanted it found. Reiner and Bertolt react to seeing it with excitement, and Ymir states they are after it. But other than that, we currently don't have enough information to decide if it is their enemy, ally, a tool, or what.
  • I've thought of a few of those points, and figured that the Titans we've seen were created over a hundred years ago by the Beast Titan. He's returned to the Walled City because many Titans have been lost in the intervening 50 odd years since his last visit due to the fight with the Shifters, and he needs more troops, supplied by the conspiracy. Ymir's past is unclear, and considering her involvement with the Shifters there are any number of ways she could have encountered the Beast Titan. I'm not sure how to respond to your comments about the lack on information; making deductions based on little information and troper Genre Savvyness is the whole point of WMG. Considering they need to locate BT at all implies they at least aren't on the same side, and since he has an intelligence of 11/10, I doubt he's working for anyone else.
  • Not sure if they even can qualify after all they've done. Even if their intentions turn out to be genuinely noble, they still are directly responsible for the brutal demise of a large portion of the human population, most of whom were just hapless civilians or well-intentioned soldiers. That may place them more into Well-Intentioned Extremist territory.

Annie's crystal prison
Is actually a form of Titan, with the same effect as the majority of Titans' forms. We know that Titans are a form of Lotus-Eater Machine that keeps their pilot trapped in a dream world. We saw Eren nearly get trapped in that fantasy of him in his house with his mother still alive. We also know the pilot doesn't age physically while a Titan. The crystal performs the same function of keeping Annie ageless (hence how she's alive at all), and based on her flashbacks, she's retreated into a dream where she's with her father at their homeland.

Annie's mother left her and her father
She probably strongly disagreed with what Annie's father wanted to train her for, and realizing it was impossible of her to talk him out of it, she refused to stay and watch her daughter be turned into an assassin, causing her to leave. It happened while Annie was very young, possibly a toddler, which is why she's only shown having memories of her father. It's also part of the reason Annie's last memory shows him in tears while apologizing to her, because his actions had caused him to lose his wife and now he was losing his daughter too.

As an above WMG mentions, Eren will be asked to eat Annie, however...
This will lead to Annie's Heel-Face Turn.

Eren still respects Annie too much to actually eat her, and/or he will strongly oppose to the idea of eating someone in general. Because of this, he will ask Annie that she helps them out with her crystallization ability instead. Something like "There's no way you can be forgiven, but if you really feel sorry then prove it by helping us." This would be both an opportunity for Eren's Character Development into getting over his black and white mentality and actually give an enemy a chance, should she express guilt (not to mention he already expressed a certain amount of compassion and desire to hear her reasons out), and for Annie to, for once in her life, "go against the flow" by defying her mission. The outcome of her being given the opportunity would lead to one of two scenarios:

1. She indeed concedes thanks to her guilt and desire to become mentally strong, and we get our first Heel-Face Turn from one of the Shifter trio. 2. She wants to redeem herself, however, feels it's impossible for her to join them after what she's done, and asks Eren that he eats her to redeem herself that way.

(And I know those are similar to the above guesses, I merely wanted to expand on them a little.)

Annie coming out of the crystal only to be killed without anyone wanting to give her a chance is a possibility too I guess, but in such case I think it would be an incredibly wasted opportunity in terms of both her character and the narrative. Imagine; having TWO Titans fight on the good side. It all depends on what message Isayama wants to get across though.

Annie will be woken up by Eren's Coordinate abilities
With the implication/theory that the ability works on people as well as Titans, the others will suggest that Eren tests it out on Annie as a means of waking her.

Eren will snap Annie out of a BSOD, leading to her Heel-Face Turn
  • Annie quiet clearly broke during that last fight. From the hysterical laughing fit she had before she transformed to her visible growing desperation throughout the fight, it all paints a picture of someone going through an emotional breakdown. So by the time she and Eren meet again, she's likely to be in pretty bad shape. It won't be a joyous reunion for certain, probably with a lot of shouting from Eren. But Eren always had knack for pushing people into standing up for themselves, and he may end up doing the same for Annie. Call it a hunch, but it seem's like Annie's been following orders her entire life, and never believed that she could stand up for her own feelings; or to put it another way, she's been pushed around her entire life (fitting in with that manga's allusions about bullying). But this confrontation with Eren will finally make her realize that she doesn't want to live like that anymore, and that she's tired of being deceitful towards people shes come to care about. To that end, she'll throw her lot in with the Survey Corps, Not to just to atone for her crimes, but to help them fight back against a world that's caused them all so much pain and torment. (Granted, that may be a little TOO Hot-Blooded, but it could proceed along similar lines)

The Shifters have taken whatever is in the basement
Last we saw them, Bertholt, Reiner, and Ymir had made it to the Shiganshina district. They were desperate to bring back something to prove they had achieved at least partial success, and since the 57th expedition had the explicit goal of reaching the basement, the three know something is in there. They may not have the key, but I doubt the Armored Titan would have trouble ripping the place out of the ground if necessary. Plus, if it prevents the audience from finding out what's in there, the author will probably do it.

The "Hometown" is located in a forest of giant trees
The is based on the "Chapter Zero", in which the human city is surrounded by massive trees the Titans won't break. The Shifters must protect themselves somehow, and all we see of the terrain in Annie's flashback is the thick foliage.

If Annie performs a Heel-Face Turn...
She will be Letting Her Hair Down as a means of symbolizing it.

When the Titan Shifters return...
It'll be because Ymir convinced Reiner and Bertholdt to have a Heel-Face Turn. Annie's conflicted enough that she'll likely turn along with them if she thinks she can get away from her orders, especially if they bring her father with them and he tells her she doesn't have to go on with the mission anymore.

     Theories about Grisha Yeager. 
Theories about Dr. Yeager and his research.

Grisha was the Coordinate, and he got cold feet.
Seeing the destruction of the Walls and what it cost him personally, he backed out of his role in the Titan Shifters' plan at the last minute, and may have injected Eren as a last-ditch effort at siding with Humanity.
  • Added possible evidence: Grisha wanted to show Eren what was in the basement after he expressed a desire to join the military. If what's in the basement is evidence of what Titans really are and the existence of Titan Shifters, he may have wanted to show Eren that killing Titans means killing innocent humans and the world isn't as black and white as he thought (and still thinks). Further, the Shifters haven't found their Coordinate, even after five years of infiltrating humanity: he's dead. Of course they wouldn't be able to have freedom to move around as trainees and look, so the coordinate would almost have to be someone who could come to them. Like a doctor.
    • Now we know for a fact that Titan-Shifting is conferred by eating a Titan-Shifter, but the Coordination ability could be, too. Which would suggest...

Grisha was a Titan-Shifter, and he used the plague as an opportunity to get into the Walls.

  • Going with the above WMG, Grisha saved the people within the Walls from a plague and that's how he earned renown as a famous doctor. It would be easy for someone like Grisha (who, notably, chose to live right on the inside edge of the walls in the very same location the young Titan-Shifters would attack years later)to be the "only survivor" of a plague in a small settlement and "return" from outside the walls because he "went looking for a cure", or something similar.

The injection Eren's father gave him is not what gives Eren the ability to Titan-shift, but what lets him keep his wits when he does so.
All humans (or some humans for some other reason) are able to Titan shift, which is hinted at by Connie's "mother" he found in his "destroyed" village.

Grisha had a role in the conspiracy, but turned against it because Carla was killed.
The basement is full of his collected knowledge of the conspiracy between the Wall Cult, the Royalty, and the Titan Shifters. He was aware of all of it and was either playing, or was meant to play, a role in the proceedings, possibly without his knowledge. His superiors called him away from his hometown because they knew it was going to be attacked when the Colossal Titan arrived, and something about the Shifter infiltration was too important to interrupt by leaving a man with the resources to route a Titan attack (the Coordinate/Coordination ability in a syringe) available to do so. In other words, he was called away for the same reason that the Scouting Legion was! He injected Eren to make him a Titan-destroying machine who could serve as an instrument not only of humanity's revenge against the Titans, but his own revenge against the puppetmasters who killed his wife.

Eren ate his father, but doesn't remember doing so.
In Chapter 53, Hange relates the experiment results to Eren. When Titan Eren was writing on the ground, he communicated something along the lines of "My father...to me." We know Eren has "episodes" when it comes to remembering his father, lending credence to the hardly-a-theory-at-this-point that Grisha has a Memory Gambit in the works (he does mention "their memories" will help you at one point). "My father injected the Coordinate to me? My father gave the Titan-shifting power to me?" Either way, something happened that night — and after the initial injection, Eren is unable to control his Titan form and eats Grisha. It would explain why nobody seems to know where he is, despite him being a distinguished doctor, and why Hanji notes Titan Eren grieves immediately after writing about his father...because human!Eren repressed the memory, but not fast enough to repress titan!Eren.
  • Or Grisha just went off to the Titan Shifter's settlement?
    • That begs the question of why he would abandon his son and adopted daughter without explaining anything to them. If he was in on the conspiracy (and he would almost have to have been), what reason would he have to escape the conflict and leave his only family behind? Keep in mind, when he disappeared, Eren and Mikasa hadn't joined the army yet; if Mikasa had been able to talk Eren out of joining the army, they could have starved to death without ever finding out about Eren's Shifting ability.
  • First part CONFIRMED.

Eren is so important because his father ran off with all the research on Titan Shifters. Ymir and her people are the original Titans that all further research was based off.
After the discovery of Titan Shifters, Dr Jaeger ran off with the remaining formula and all of his notes. Eren is so important since he's the latest "model" as it were and someone talked about the key around his neck. Many of the Titan Shifters were close friends and comrades of Eren and at the very least, they may have heard rumors that he knows how to access his father's work. The Titan Shifters also seem keen on acquiring Ymir who is patently unaffiliated with them, mainly because she may be a live sample of the original Titan or Titan Shifter line.

The plague that Dr. Jaeger saved mankind from was the agent that makes Titans and/or Titan Shifters.
The Titan Shifters either are, or are descended from, people who were exiled because they had or carried it. The Sasquatch Titan is basically spreading a disease, one that Grisha was able to identify, but not cure: the closest he could come was a vaccine, which is what creates Titan Shifters. They're immune to becoming regular Titans because they can willingly become Titans! Grisha already knows that whatever Mikasa's got going for her will protect her, so his last act before fleeing the Walled City was to immunize his son.

The serum Eren's father injected him with is the perfected formula of what originally created the Titans.
As the Rogue Titan, Eren is pretty much a unstoppable killing machine, utterly annihilating Titans left, right, and center, and not even slowing down when he loses a limb due to astoundingly fast regenerative capabilities. He can even match the Colossal, Armored and Female Titans blow for blow. So why does his form have absolutely perfect physique and performance? The serum his father injected him with is the perfected version of what created the Titans. Whatever the Titans, it is highly likely they were originally human at some point, further confirmed with Ymir and the titan in Connie's abandoned village. The malformed and mentally retarded Titans may have been test subjects for a serum designed to turn them into supersoldiers.

Grisha was studying memories, not Titans, and discovered the concept of Titan Shifters by accident.
The concept of memory seems to come up a lot: Eren "remembers" Mikasa's hair being short, he knows how to trigger his Titan transformation but doesn't know how he learned, he gets lost in his dream of home and has to remember the real world to get out, Grisha even talks about "their memories" when he injects Eren. It's possible that he found a way to take memories from people, and just happened to get a sample from a Wall Cultist or Titan Shifter, learning about the possibility of humans that become Titans. Injecting Eren with literal memories of that concept gave him the necessary knowledge of how to change back and forth.

Eren is a Titan because his dad is too.
  • While the "Mr. Jaeger made his own son a Titan through science" theory seems more plausible, I believe that the creation of Titans may be more of a biological thing; i.e. "THIS POWER HAS BEEN PASSED DOWN FROM GENERATION TO GENERATION" kind of thing. Bertholt, Reiner, Ymir, and Annie were shown to be able to transform, the former two especially, at such a young age, so for it to be passed down from birth could be possible. Problem with this though, is that if it were the case, there would be a good chance that Connie would be a Titan too, based on the attack on his village.
  • Assuming that the second ending of the anime is as big of a spoiler as we think, there is a history of Titans attack as such against something other than humans. But that's not the important part; what's important is how long ago does the history predate to and whether or not the Titans existed before the creation of Mr. Jaeger's injections.
    • Grisha Jaeger's injections couldn't possibly have predated the Titans (if only because there's no way he's over 100 years old, even Titan Shifters age normally), unless they aren't his to begin with, in which case Titan-ism wouldn't be a hereditary trait at all because the formula would have to have come first. Being children after becoming Titan Shifters doesn't lend any evidence either way, it just means they were Titan Shifters as children (like Eren could easily have been, if he'd had the right trigger).

Eren ate Grisha.
Grisha was, presumably, alone with Eren when he injected him and turned him into a Titan, but what do Titans do on instinct? They eat humans, and Grisha would have been the only one around! Following that...
  • In Chapter 53, Eren shifts into his Titan form for experiments, and then grieves horribly after writing, "My father was _____ by me". Eaten would fit nicely.
  • The Chapter 53 line is a mistranslation. More accurately, it states that his father did something to him.
  • Both confirmed.

Grisha committed suicide-by-Eren.
Grisha was a titan shifter all along (or at least for some amount of time before the fall of Zhiganshina), but was too overcome by grief after Carla died to continue doing... whatever he was doing. So he passed on the Shifter power to his son in the hope it would give him an edge, by turning him into a regular titan and letting it eat him.
  • The part about Grisha being a titan shifter seems to be confirmed.

The Central Brigade was after Grisha, and he is either dead or in custody.
Grisha was a doctor living on the very edge of human territory, a place considered undesirable to live. But on the day that Shiganshina fell, he had been summoned to the Interior on some kind of important business and it seems as though this was not his first journey there. We know in the week following the loss of Wall Maria, the Reiss family was in a state of panic and the Central Brigade was doing damage-control and "clean up" operates such as the one that ended with Historia's mother being killed. During this same time, Grisha took his son out into the woods and gave him some of injection while behaving like he'd never see his son again. It was because he knew the Central Brigade was after him, and all he could do for Eren was give him the Titan's Power as protection and then get as far away from him as possible. Since then, Grisha has either been captured and held prisoner by them or was assassinated.

     Beast Titan Theories 
Theories about Monkey Trouble himself.
Please Note: Theories about the Beast Titan being any particular person within the Walls have already been Jossed by its unfamiliarity with 3D Maneuver Gear.

The Beast Titan is Gaias Revenge.
It is in fact humanity's true enemy, but it doesn't want to wipe them out, per se. It's a being that knows that humans could potentially overwhelmingly overtake the planet if not kept under control, so it has the ability to turn humans into the ultimate population control mechanism, as well as something completely harmless to the environment: a creature that does not care about attacking animals, needs only sunlight to survive, and will only consume humans. It decided to sneak into the Walled City when it looked like the Titan Shifters, who lacked their protection, were going to allow humanity to spill into the world be destroying the Walls.

The Humans that can turn into Titans were intended to be weapons against that "Thing"
In Chapter 35, a Sasquatch like creature appears. It is nothing like the Titans seen in the series thus far, and it even defies the standard height limits of normal Titans by being 17 meters tall. Given its command over the Titans, it is still affiliated to them, but not them. To top it off, it KILLS a Titan that does not listen to him. Perhaps that "Thing" is one of the creators of the Titans, and possibly one of "Ymir's People".
  • The Sasquach Titan is confirmed to have been their original goal. Why they'd be willing to abandon it in favor of making off with Eren is still up in the air, though.

The Beast Titan is the Big Bad behind everything.
This is linked with the supposedly leaked story behind SNK, which (MAJOR SPOILER) talks about a professor creating Titans to exterminate humanity because he thought they had become too religious (a group of religious zealots were against his researches and tried to kill him, but ended up killing his family instead). As a result, he created the mindless Titans, but also gave himself and a group of chosen followers the ability to become intelligent Titans. Going out from this, the Beast Titan is the professor in his Titan form, which explains why he talks, can create other Titans, and has an intelligence rating of 11/10. The spoiler also mentions that he's unaware of how humanity managed to secure themselves somewhere, so it makes sense that he didn't know about the 3D maneuver gear. This is all based on whether or not the leaked story is true, however.

The Beast Titan is a Shifter, too.
While it does seem impressed on how humans created the 3D Gear, he could still be human, just from another part of the world.
  • At least partially confirmed. It refers to itself as being "in the neck", confirming it has a pilot. Whether said pilot is human or not, now that's the question.

The Sasquatch Titan is humanity's true enemy and can turn normal humans into titans:
Judging by Chapter 46, it seems Reiner and Bertholdt true objective was related to the Sasquatch Titan. When Reiner told Eren about their identity, he said they wanted to destroy every human living inside the Walls, but now everything changed and they are willing to let them live, as long as Eren goes with them. The reason everything changed is because they saw the Sasquatch Titan, which was their primary objective. See, normal Titans are dangerous to Titan Shifters as well. And judging by Connie's village, it's safe to assume normal Titans used to be normal humans. It would be a problem to the Titan Shifters kind if every human alive turned into a Titan, something the Sasquatch Titan could do. They knew he could do it, as he was the one who created the first Titans, but didn't know where he was to hunt him down, since he was probably hibernating or something like this, as he was not aware of mankind's 3D maneuver gear. As they couldn't kill the Titan who could create more Titans, they decided to kill every human being instead, preventing them from ever transforming. But now that he is awaken and started the cycle again, their objective changed.
  • Wait, so Reiner and Bertholdt's story about their village being attacked could be true, and they are of the few survivors? That could be why Reiner is so torn over guilt, maybe they had a logical reason so get inside of the Walls, maybe related to Sasquash Titan or the Wall Cult/Titans in the Walls thing, an option to a Final Solution. But they were young, naive and uncaring about humans, so they did what the first thing that crossed their minds and the one that was the simplest, kill all humans. But then they got in, started to interact with humans and understood how horrible were their actions (at least Reiner and a bit Annie, as Bertholdt has Lack of Empathy as big as the Colossal Titan). That could explain why Ymir is actually hairy compared to the other three, she is (or was) part of Sasquash's faction and did attack Reiner's village. Reiner and Bertholdt's surprise is not about being attacked by a Titan Shifter, but about Ymir being that one.

Theories on Beast Titan.
It probably experiments on humans because reasons unknown to reach some unknown goal. Maybe to restore his own kind since it is the only one of such type we saw by far.

The Beast Titan(s) will come back with a counter-measure to the 3DMG.
That bit about humans needing a common enemy to unite against isn't just flavor-text for the setting; it's foreshadowing. The Titan Shifters and the Humans will be forced to unite against the Sasquatch Titans.

The Beast Titan is a natural Titan, distinct from the rest.
We now know of the Beast Titan, it looks like a massive ape with long arms and is incredibly strong, eloquent to the point of understanding and speaking human speech, and even gives lesser Titans orders. But it isn't a Titan Shifter. It is a genuine Titan. The Beast Titan might be the remnant (or vanguard) of a natural born race of Titans, which Humanity discovered shortly before their near-extinction living in remote areas. They may have experimented on them,drawing plasma from them and testing its capabilities in humans, possibly in the hopes of creating a super-soldier or virus. From there, the first batch of subjects exposed to the serum would have malformed horribly, to the point of their limbs becoming disproportionate and their size increasing, all the while their mental faculties are completely erased except for their desire to kill the enemy. They spread, able to expand their numbers by eating enough people to form a new Titan, causing the near-extinction of mankind and making them flee across to sea to presumably Europe, and discover the Walled City. The Beast Titan may have resurfaced as the last of its kind to enact its retribution on Humanity.

The Beast Titan is humanity's harvester, and enemy of the Shifters
The Beast Titan is responsible for harvesting human beings (provided to him by the conspiracy within the government), transforming them into Titans as weapons against the Shifters' faction. This fits in with the whole "humans as livestock" theme. The reason the Shifters sent three kids in to destroy the Wall is because they're on the losing side, short on manpower, and need to cut off the Beast's source of Titans, then win by attrition with their ability to reproduce naturally. Reiner and Bertholdt were trying to capture him as an alternate way to eliminate Titan production, since if it was on their side, trying to capture it makes no sense. BT's test of humanity's strength was really just to see if the people within the Walls are a credible threat compared to the Shifters. They aren't. To fill in the rest of the gaps, I'm betting "Ymir's people" are a group she shared the "Titan's Power" with after stealing it from the Shifters, and Grisha was a part of the conspiracy working to undermine it by obtaining Titan Shifting for the people within the Walls to protect themselves.
  • Unlikely that it is the Shifters' enemy. Bertolt explicitly mentions that if Eren turned out to be a dead-end, they would have to return with Annie, Krista, and "that thing" (referring to the Beast Titan).

The Beast Titan used the 3DMG to design the Anti-Human 3DMG, seen in Chapters 57 and 58, as he is allied with the Wall Cult and the corrupt Military Police.
The Beast Titan encountered Mike, and after some chatter, removed a piece of his gear, and left him to be eaten, correct? We already know that the Beast Titan is intelligent, and is (presumably) capable of creating plans. He took the piece of the 3DMG, and came up with a design to use it's mobility and weaponry against humans. Because humans are constantly trying to keep distance unless necessary, he used the idea of guns, which he likely has witnessed at some point in his life. He then chose to put the grappling hook mechanism on the arms, for more accurate mobility. He gave this design to the Military Police, who then actually began mass producing them to combat their threat: the Scouting Legion.
  • Two major problems that make this almost impossible. First and foremost, it's only been two weeks since the Beast Titan appeared. That isn't enough time to reverse engineer, work on a completely new concept, create prototypes, and then have a fully-functional and mass produced version. It takes 3 years of training for recruits to use the standard gear, learning a whole new system would take much longer than two weeks. The second issue, obviously, is that it headed out to the Titan lands.

The Beast Titan was the first Shifter.

Animal motifs are a big deal in this story; the beast titan looks an awful lot like a gigantic ape, so it might be a hint that he is an early "ancestor" of modern Titans. It would also explain how he's so knowledgeable about Titan behavior (he may have been a scientist who helped create them) but completely ignorant of human technological advancement, and why he knows how to turn humans into Titans. Annie, Reiner and Bertholdt were looking for him because they come from a Titan-shifter-friendly place that presumably has more Titan-shifters, and if the Beast Titan were to find it, he'd feed the entire population of shifters to his own pet Titans. Considering one of Bertholdt's childhood flashbacks, the Beast Titan may have already done it once before.

     Oddball/Crack Theories 
Unique theories! Crossovers, crack, and assorted other random stuff!

Eren and the Coordinate
It seems likely that the substance Eren was injected with was NOT the Coordinate. Rather, it was what made him a Shifter. Why? First of all, Mikasa. There's enough evidence to suggest that her abilities are tied to Eren's nature, given the constant brain flash montages during the fight with the kidnappers, as well as just before Titan Eren makes his debut. Remember, Mikasa's rescue took place before the injection. Presumably, anyway; it's hard to tie down the precise timing of these things since all we have is a dream sequence. Also, remember Grisha's words: "Get to the basement. This power will help you then [...] their memories will help you use it [either Coordination or Shifting]." Grisha just made him a Shifter to ensure he would survive long enough to be able to use his innate Coordination abilities. The bit about memories is a tad more ambiguous. It could mean that the memories of other Shifters somewhere, which he could access with his mental link due to being the Coordinate, would teach him how to Shift. This has some evidence backing it up, like Eren not understanding how he knows to bite his thumb...

The nature of the Wall Cult
It's been long assumed that the Wallists were part of the internal conspiracy, but I'd say that as of right now that's pretty well Jossed, given Nick's fate and how his character was generally portrayed. One possibility is this, going from the assumption that Shifters were created as a weapon by humanity: when the first war ended, humanity agreed to remain trapped within a prison, and to destroy their Shifter army by killing them and using them to create the Walls. But they lied. Instead, the Shifters were kept alive, but dormant, waiting for the time to come, and the Wallists were founded to watch over the army and keep it a secret from Sasquatch and co. That's why they're so terrified of "desecrating" the Wall, and why they refer to the Walls as a gift from God to save them from the Titans: they ARE humanity's saviors, just not in their current form as Walls. And if the Shifters were to wake up prematurely, the Titans would learn the truth and destroy humanity once and for all. The only remaining questions are A. who IS the enemy within the Walls, and B. what are the motivations of the Hometown Trio and their allies? They don't seem, as I once speculated, to be affiliated with the Wallists, or Annie would have been careful not to damage the Wall as she did. But their goal is clearly not to outright attack or destroy humanity, because that would have been trivially easy: destroy Shinganshina, destroy Trost, destroy Stohess, humanity's done.

The Walls are a three-layered Transmutation Circle
Weird cult, government conspiracy, super-powered humanoids... it all fits!

Mikasa kicks so much ass because...
We are told in the story that she is the last person with Oriental blood in the world. Her name is clearly Japanese. Her mother, in the brief time we knew her, told Mikasa that her family has skills that have been passed down for generations. Conclusion: Mikasa is a Ninja descendent. Therefore, she is the last ninja in the world, and thus, she is THE walking, talking incarnation of Conservation of Ninjutsu to the extreme.

Mikasa is not a ninja, but...
She is a descendant of Ryuu which would give her blood of Hokuto and (possibly) Nanto

Additionally, the reason why Asians such as Mikasa are valued so much by slave traders is...
..that the ancient Asian race and arts of Ninjutsu were once considered by the Government to be the last hope against the Titans. The development of 3D Manoeuvre Gear and its arts of leaping around through the trees and rooftops were more clearly inspired by Ninjutsu, rather than the classical European style of building bigger cannons and bombs and winning through brute technological force instead. The original plan wasn't to sell Mikasa to just some rich perverts, but rather for a Super Breeding Program. The aristocracy took note of this and thought that the Asian Super soldiers are going to replace the old useless European race, and thus these "rich perverts" wanted to kidnap and breed with Asian women so that their aristocratic property is still preserved even when those bloodlines have Asians with them. Eren interfered with the kidnappers' plans, and the Titan Shifters provided an alternative to the supersoldier program.

Master Xehanort is the real villain
He personally trained all the Titan Shifters. We see it happening first hand at the start of the series!

The series takes place in the future of Ao Oni
  • People who die within the confines of the oni's house rise and become oni themselves, each having a misshapen and Uncanny Valley look that highly resembles most of the Titans. It's possible that the oni were the first proto-Titans, and they can define pieces of territory such that any human that dies there turns into an oni/Titan themselves. Any changes in size and pigmentation could be the result of the Titans being several generations removed from the original blue oni.
    • One 3M Class Titan in particular highly resembles the Ao Oni. Furthermore, author Hajime Isayama has linked a Japanese Let's Play video on his blog in 2011.
    • This could explain why the Asian population has been heavily decimated, Japan was ground zero.

This world is in the same universe as Tengen Toppa Gurren Lagann
  • On another world, faced with the growing technology of spiral powers and waist mountain grappling hooks, the anti-spirals, or even a spiral warrior placed the Titans to contain their race in the three Walls. Whenever a spiral warrior, also known as a Titan Shifter, arises, the spiral king of this world sends his men to capture them.

Titans are Witches/Warlocks and/or their Familiars.
Surprised that one hasn't come up yet.
  • Unlikely, considering this is a somewhat sci-fi setting the injections being the lead on the whole biological weapon concept.
    • Not as unlikely as you might think (outside the meta, anyway). Madoka's wish unmade and then remade the universe. There's nothing to suggest that another magical girl couldn't do the same, or that Madoka was the first magical girl to do it. For example, some particularly savvy one might have wished for humans to have a Barrier, too, and it resulted in the Walled City. The equivalent despair couldn't be contained by just one Soul Gem, so it was released into the universe as the Titans we know, and the Titan Shifters are the Puella Magi of that world.

Mikasa and Levi will turn out to be human-sized Titan Shifters
With miniature 'pilots' in their neck. And Eren will discover Mikasa's 'pilot' after she seemingly got eaten by a Titan and carry her in his pocket for a Crowning Moment of Funny.

Shingeki no Kyojin is a sequel to Watchmen

This is based on Isayama's statement that Erwin and Levi are inspired by Ozymandias and Rorschach, respectively. Yeah, I'm pretty sure he only meant in terms of character design, but stil, it's fun to speculate. Warning, spoilers for Watchmen abound.

Ozymandias' plan with the squid failed to create world peace. People quickly forgot about it; the threat wasn't constant and deadly enough. Undeterred, Ozy created a new monster that would more actively attack humans, existed solely to attack humans, and could spread across the entire world. Thus the Titans were born The first Titans were more beast-like and intelligent, such as the Beast Titan, but eventually Ozy discovered a way to create Titans from human beings.

The Titans were a resounding success — too successful. The Beast Titans rebelled against their creator and began creating human Titans of their own, at a much faster rate than Ozy had intended. They decimated humanity before humans could get a chance to truly unite against them. Realizing he'd made a mess of things, Ozy used genetic engineering to keep himself young and immortal, and lived for centuries trying to come up with a solution to the Titan problem. In this current era, he is living as Commander Erwin Smith of the Survey Corps. And for help, since he can't do this alone, he created a clone of Rorschach using Rorschach's genetic material, and that clone is Levi. Levi doesn't remember being Rorschach and has no idea Ozy created him, which is all the better for Ozy/Erwin. He just needs Rorschach's skill at cutting things up and cold-blooded killing. I don't know if the Shifters are something Ozy/Erwin is responsible for, too.

The series will end with Doctor Manhattan deigning to return to Earth and fixing the Titan problem with a deus ex machina.

Instrumentality
  • The three Walls were created by the Wall Cult to form a massive Transmutation Circle...which will transform all the humans and Titans iunto a giant Rei Ayanami. Don't tell me I'm the only one who's thinking it.

Heterosexual People Cannot Become Titan Shifters
Ymir is a lesbian, Bertholdt and Reiner are bisexual, Eren is asexual, and Annie could be any of the above, considering her lack of romantic interest in anyone. This is why Dr. Jaeger didn't choose Mikasa to become a Titan Shifter, despite her being smarter and a better fighter than Eren; she's straight. Likewise, all of the normal Titans we see are straight people, who can never become Shifters.
  • That may lead to some Unfortunate Implications, considering four out of those five are trying to kill humanity.
  • Wait, when was it ever confirmed that Bertholdt was bisexual? He was never shown to be attracted to any men and Reiner teases him for having a crush on Annie. I know that Bertholdt/Reiner/Annie is a very popular pairing, but let's not jump to conclusions here. Also, a small nitpick, Ymir is less trying to kill humanity and more saving her own skin.
    • There's a particular scene where Reiner teases Bertholdt about how Bertholdt is always staring at Annie and so he knows Bertholdt has a crush... but when he says that, Bertholdt is very pointedly staring at Reiner, and is often staring at Reiner whenever they share a scene. There's a lot of reasons for this, but saying that he's never been shown to have an attraction to a male character is a little bit of a stretch...

Attack of Titan takes place in the same universe as Judge Dredd
Everyone is living in the ruins of the old world after everything got nuked to hell in massive cities. The people in the Walls is another city except they forgot about the past and regressed technologically/or the government is lying and repressing them. The area the story takes place in was hit less hard by nuclear bombs and is thus able to support life again, compared to the wastelands of Mega City-One. The Titans are a new breed of mutants that have a hatred of regular humans from not letting them in hundreds of years ago.

So everyone wants to leave the Walls of the city in Attack on Titan because it can actually support life, the order of the Wall though know the ugly truth though that although their area is nice the rest of the world is a wasteland.

Armin is He-Man
Possibly his parents returned back to their planet. When he was old enough his parents took him and raised him to be Prince Adam.

The Titans have another weak spot besides the base of their necks
In Chapter 9, Reiner makes a comment about shoving a sword up their asses, as it's the second of only 2 weaknesses they have. Of course, we haven't actually seen that happen, so for all we know it might actually work.

Titan Shifters have no genitals in their human forms as well
There are currently five Titan Shifters we know about. Four out of them prefer to hide their abilities, and therefore will not mention this fact. The fifth one is Eren, who is young enough (15) to have never seen a naked human before. This is also why Eren ignores Mikasa's advances, and why Ymir only expresses her feeling towards Historia by touching her cheek. Titan Shifer love is platonic, they can never get past innocent fondling. One problem with this theory though, in many armies, there are shared showers, which would have been a chance for Eren to see another naked male, or vice versa. It is however possible that customs are different on the SNK world.
  • So, since it's already pretty much established that (at least most) Shifters are regular Titans who regained their sense of self, and those regular Titans are in turn normal humans who were turned into them, it would mean that being turned into a Titan somehow vanishes your genitals even in your human form?
    • It could be quite poetic, to get your "second life" as a Titan, you must give up the ability to create life.
      • Wouldn't Eren have noticed his junk went missing? He didn't start out as a Titan Shifter, after all.
      • He would if it happened after his first transformation in the manga, but it's hinted that Eren has been a Shifter longer, after having been injected with something by Grisha, and his memories about it are unclear, so it is somehow possible he forgot how his body looked like before. The biggest problem with this theory is still the lack of privacy in the army. Even forgetting Eren,someone should have noticed it. Wouldn't there be physical check-ups or the like?
      • How do we know he didn't start as a Shifter? Maybe he did and was adopted. For all we know, his dad's injection could have delayed shifting, or given him his "coordinate" abilities.
      • We know he didn't start as a Shifter and wasn't adopted because:
      • He has a strong resemblance to his mother. Adoptive parents love their kids as much as any biological parent, but he looks exactly like her, that's not a coincidence.
      • His lack of interest in Mikasa (or her YMMV advances) simply boils down to her being his adoptive sister. It's made pretty clear that Eren views and treats her like a sister, and despite what some shows/books might portray, the vast majority of adoptive siblings look at each other the same way as regular blood siblings. He's just not sexually attracted to his sister.
      • If his father's injection delayed the shifting, Eren wouldn't have any reason to not remember it happening. Grisha wouldn't have had to tie him down or be insane if what he was giving Eren was treatment for a condition because he's a doctor, he does that kind of thing all the time. It wouldn't have been unusual enough for Eren to block out the memory and only half-remember it in a nightmare, and it wouldn't have called for Grisha completely disappearing from his life for no apparent reason.
      • If he had been a Shifter when his mother was killed, he would have shifted to save her. He shifted the first time because he had a breakdown over realizing he had finally lost everything he'd gained since his mother had been killed. When Carla died, he had the same thing happen. Since we know the thumb biting/blood loss thing is instinctual, if he had been capable of it, he would have done it then.
      • Some things in Sn K army are very different from other armies, for example, the fact that the best troops can do police duty instead of combat/guard duty, in most armies this is the opposite, there are special high class combat units, not police ones. Maybe this is different as well? I admit this is beginning to look like a stretch, though.

Titan origins, most of the Humans outside the Walls didn't get killed by the Titans and were used to farm the Titans today, The three goddesses of the Walls are the villains and the Wall Cult is to keep this a secret, and Grisha Yeager is the Beast Titan
  • So, we know that when the Titans appeared, they started eating people. But you see, if the Beast Titan was able to turn humans into Titans, then where did the Titan shiftes come from? What if the three goddesses, Maria, Sheena, and Rose are the real villains and turned a select few people into Titan Shifters while turning a massive group of humans into Titans so they can eliminate humanity? If they're goddesses, then they should be able to break the laws of physics, because the properties of Titans go against the laws of physics, and in particular, mass-related laws and the Square/Cube Law.
    • Maybe there were prototype Titans who were made so that they broke the laws of physics to be more dangerous.
  • What if Grisha Yeager, Eren's father, is the Beast Titan? The Beast Titan is the only Titan we know for sure that can turn other humans into Titans, and Grisha Yeager was the one who gave Eren the injection that turned him into a Titan Shifter.
    • He was "acting weird since mom died" according to Eren and was crying while he gave Eren the Titan injection, so maybe it was because he felt guilty that he's related to all this.
  • What if the humans that died when the Titans first showed up weren't actually killed, but used to give birth to more children for Titan farming?
    • It's a bit strange that all of Asia was eaten, but other races still exist.
Sasha is Latvian
In Latvia, no potato to give.

The Titans are koloss.
Albeit ones with the ability to burn Feruchemical gold. Their spikes are clustered at the back of the neck. (Also they're made from the Hemalurgic essences of a lot more than...what was it, five humans? Either that or they're really, really old, at least for the big ones.) They eat humans because...I dunno. They're just like that. Oh, wait, no. There's a super-powerful Soother out there somewhere, probably affiliated with Mr. Sasquatch, controlling them all, which is why they never kill each other. And Titan Shifters have something to do with Feruchemy. The six-meter in the side story talking about Ymir, and some of the theories surrounding it, really reminded me of Human and his interactions with Vin.

Marco is the real head villain
Not particularly likely, but there are several things that are a little... strange. Though, if anything, he would be an extremely Affably Evil villain.
  • Annie never elaborates on the circumstances of his death, only claiming to have "found" his gear somehow.
  • His death isn't shown on-screen.
  • His body isn't found until two days into the clean-up, and given its state, could have even been faked. Jean wouldn't want to look too closely, after all.
  • It's anime. It wouldn't be the first time that the harmless, nice background character that died to advance a more involved jerk's characterization ended up being the main villain.
  • There's been a lot time spent on him early on, but nearly none afterwards. Granted, this could just be for pacing, but it could also be to lull the readers into somewhat forgetting his importance.
  • ...Yeah, I got nothing else. But it would be a rather big twist, especially after all the Titan Shifters that have been introduced.
  • Huh.
  • This might be Jossed by Annie's visual novel, where she monologues that she caused Marco's death as well as the other Trost victims. For the theory to be correct it would mean his mission is unrelated to the Shifter trio's.

Jean and Marco are both Titan Shifters not associated with Reiner, Bertholdt, and Annie
Because it would be hilarious if it turns out that all of the very top students sans Mikasa were Titans. Also, of all the remaining human trainee soldiers, Jean and Marco are the only ones whose pasts have not been revealed or hinted at. This was why Jean was so surprised that Marco had died since he could have shifted into a Titan and saved himself. Or alternatively, he revealed his Titan self to Annie and she killed him, cutting off the side of him that had Titan markings to hide his secret.

The Titans were created by whoever created the Angels
You heard right. Whoever created the Angels eventually decided to create the Titans. This is assuming the events of Episodes 25 and 26 of Evangelion are the canon episodes in this series and not End of Evangelion. The Titans were created as much more aggressive beings than the Angels, and gave them a taste for human flesh. The original Colossal Titan (before the DNA was forcibly injected into Bertolt) defeated the Eva units, killed yet another Rei clone, and forced Shinji and Asuka into hiding while Gendo hunted them down for their failure. This led to the two of them to send another Rei clone into an alternate dimension, a clone who adopted a new yet similar look and the alias Yuki Nagato. This clone was meant to lead the last of humanity to a new home, but was sidetracked by Haruhi. Meanwhile, the world fell into ruin, and efforts by NERV and SEELE to stop the Titans were in vain. 800 years later, the Titans are now the dominant species, and this led to the events of Attack on Titan.

How does the Rei clone/Yuki fit into this? A small portion of the DNA used to create the Rei clones was stolen by the aliens and used to create the Titans. No one is sure how, not even Shinji and Asuka, but somehow, the aliens pulled it off.

Isayama was inspired by a boxcutter
He was going to just make the iconic weapons a pair of katana but then he saw boxcutters on his table and decided to swing them both around in his room making "whoosh" noises as he pretends to kill titans. I'm just waiting for him to admit it on camera.

The leaders should've studied Sasha to understand the Titans
Sasha strikes one as a little odd with her fixation on food and eating. Not only is she a cloud-cuckolander she had a rather weird way of describing why she ate the potato, Providing the potato a safe shelter in her stomach. Perhaps if humanity studied her habits and psychology, they'd have a better time understanding the psychology of the Titans and why they behave the way they do.

Once Eren learns to fully control his Titan form, he will get a set of giant 3D gear.
For no other reason than the fact that Rule of Cool demands it.

     Other/Unsorted 

The Titan that attacked and ate that other Titan in Chapter 39 did so because its victim's "pilot" was slightly more intact than your average Titan.
In the opening pages of Chapter 39 we clearly see a Titan shove another to the ground and then begin violently attempting to dismember it, ripping off its ear and pulling at is hair. Now that we know Titans target Shifters as they primary prey, because eating them returns them to their human form, it could be that the human inside the attacked Titan hadn't been completely absorbed yet, thus making it the closest thing to a Shifter the other Titan had encountered, and it went for it.
Rod Reiss isn't going to eat Eren himself. He's going to have Historia do it.
In order to gain Eren's Coordinate power, he would have to turn himself into a Titan and then eat Eren. If something went wrong (such as Eren escaping, Scouting Legion interference, etc), he would be faced with being a dangerous monster forever until someone kills him. No matter how much one wants power, the risk is just too high. It makes more sense for Reiss to offer it to Historia so that she will become a Titan and devour Eren, gaining the Coordinate power. Then, he has the power contained in someone who is implicitly close to him — his daughter. Her father could offer to recognize her as a legitimate heir or simply to provide the love and belonging she's been looking for her whole life if only she sacrifices her humanity and one of her friends. With the resources the Reiss faction has at their disposal, it's not unlikely that they have access to at least one sample of the Titan serum, or whatever it is that makes humans into Titans in the first place.

Kaney Ackerman is a Boxed Crook.
Levi mentions that he was a notorious Serial Killer, and he seems to have specialized in military police, as he killed over a hundred of them and was never caught. However, something happened, causing him to act as an extremely deadly enforcer for the monarchist conspiracy. It could be that he was caught, but those pulling the strings behind the scenes saw a use for someone with his, er, skillset, and promised him a stay of execution if he joined their group. Whether or not he's come to believe in their cause like Sannes, or is just a Psycho for Hire remains to be seen.
  • Partially confirmed; he was a genuine serial killer before being asked to join them to fulfill a mysterious "dream" of his.

Geographia is Eren and Historia's daughter from the future.
somehow she manages to find a way to travel back in time.

Levi and Mikasa are related
The reason why they kick so much ass is that they're the last Asians on Earth who have inherited the Ninja gene. Therefore, they are THE walking, talking incarnations of Conservation of Ninjutsu to the extreme. They also have a striking resemblance, maybe it's just The Stoic demeanour, and the narrowed eyes characteristic on Asians are more apparent in him.
  • Their being related to each other is strongly supported in Chapter 56 when we learn Levi's last name is Ackerman.
    • But Levi may be only related to her via her paternal side. Her maternal side is the Asian one. I think he's Jewish-German or something.
  • When Mikasa was younger, we see her mother bandage up her arm after tattooing the traditional symbol of their 'clan' onto it. The cover for "A Choice With No Regrets" shows Levi wearing a similar bandage on his arm...

Alternatively, some Japanese survived through the Titan crisis, and the characters never knew about it
Japan has ninjas (better suited for 3D Maneuvering) and it's on an island (less risk of being crossed by Titans), so why not? Also, the super-sharp blades used for cutting through Titan flesh? Clearly based on the katana.

The story doesn't actually place on Earth.
It takes on planet similar to Earth that was colonized by humans in the past since Earth was either getting destroyed or overpopulated.

Other Humans may still exist
We know that they say most of humanity was eaten, but what about areas separated by oceans and seas? The idea of having other races seems to been forgotten, so most likely any kind of contact between nations separated by sea is gone and they simply forgot those countries existed. It is possible Titans did not appear there or they made Walls like the main series.
  • Since it is 9th century Germany [Due to Eren Jaeger's name being derived from the German phrase [roughly translated in English: They are the prey, we are the hunters]. While Asia itself was established to have been wiped out in Mikasa's flashback, there is no reason to doubt the ability of the Native American tribes to actually transform into Titans.
  • Also, settlements in steep mountains. It's not impossible, and think about how narrow and steep some of those paths can get. Titans simply wouldn't be able to get up there, so the humans have another safe haven in that sense.
    • The locations part of this is Jossed, they go over it in the manga. Titans don't need paths and most of them are pretty good climbers, if they can climb at all; the villages in the mountains were hit first, and the original humans to settle the Walls had to go across the sea to do it. Titans exist wherever there are humans.
  • Well, in terms of mountains, if they were settlements on something like, say, the Anhui Province Mountains, then the cliffs would be completely sheer at points, and thus safe from Titans. We know that it's possible for primitive people to scale them, because people did scale them and carve statues into them.
    • There is no place safe from Titans. Titans are people. If there are people, there are Titans.

Mikasa will become a Titan
Not a Shifter, either. She's already tapped into her brain for absolute control. She's been getting head pains that may not have been from wounds (I can't see any). Something will happen that will trigger her transformation into a Titan. As suggested by the Beast Titan, they too have humans in the napes of their necks. Being enveloped too deep in emotions bonds you with your Titan (think back to Eren's smiling face as he recovered from his first Titan transformation), making you irretrievable and indistinguishable from the flesh in the neck. Mikasa freaking out about Eren will become a Titan this way.

Ancient WMG — Mikasa is somehow part Titan.
Concocted after reading the first few chapters, now completely defunct (maybe). It would've explain her bizarre strength (as it didn't seem to be "comedic anime girl strength"), plus her (at the time) unexplained parentage — because no one is ever adopted in fiction unless they have a mysterious plot-relevant past. I held onto the theory even once her backstory was revealed, because honestly, who can break a floorboard just from the pressure of jumping off it? Maybe it was the result of some crazy science experiment, or maybe Titans actually had some way of reproducing, and she was half-Titan or Titan-in-human-form? Of course, now that everyone and their mom is turning into Titans, this may still come true in a round about sort of fashion.
  • I've reread the chapters, and I'm inclined to agree. There was a panel which shows her brain doing...something, and she might actually be a perfected weapon of some sort. Also that number carved into her by her mother is very suspicious. At the very least, Mikasa has been genetically modified.
    • It might also explain why Titan Eren attacked her the second time he changed, the same way the Titans tried to eat Eren even while he was in Titan form...
    • It could also make sense if Grisha had something to do with it. There's no way Mikasa would ever allow Eren to be experimented on... but she might not interfere if she could be assured it would work by offering herself as a guinea pig first.
  • Could also be why Dr. Jaeger visited Mikasa's residence when she was young.
    • There's not a whole lot of proof to back this up, but I think it's weird that Mikasa and her mother were living in a place full of people with Germanic surnames but were still the last of their kind. The revelation that the Walls were found by a bunch of aristocratic refugees only makes it weirder that there aren't more "Oriental" people around. Could be she's not the victim of weird science, but descended from people that were?
  • Something also bugs me. Looking back at the beginning of the manga, Mikasa gets some weird headache and then the same thing happens in Chapter 45. Perhaps this could be a memory block like how Eren forgot what his father did to him before his Titan transformation? Maybe Mikasa was experimented on as a child and just doesn't remember.
  • I'm going to have to chime in an agreement with this theory, having just skimmed through some of the earlier chapters of the manga. Note in part 15, pages 9 and 10, the trainer is giving everyone a hard time at the beginning of the training academy with the exception of those who have "already been through hell — you can see it in their faces". Take a close look at exactly who he's not screaming at. With the exception of Mikasa (that we know of), every single one of them is a Titan Shifter.
  • Also, when Eren couldn't control his Titan form, he inexplicably went out of his way to focus on Mikasa, ignoring every other human. Could it be possible that he can sense Mikasa being a Titan?
    • I had always thought that he was annoyed at Mikasa being protective over him right before he transformed, and that affected his goal. Going from "Move the rock" to something along the lines of "Man, I want to smack Mikasa and shut her up."

The purpose of the three Walls are not unlike that of the settlements in Claymore
The Anachronism Stew that is the 3DMG and the blades used by the military hint that mankind either has or had a much higher level of technology than they do now. Combining several other theories, the humans living within the Walls are subjects of a biological war machine experiment not unlike that seen in Claymore. There are other humans out there; these poor souls are actually an isolated community used to test and refine the Titans. The Titan Shifters are just the latest breakthrough.

Time Travel is involved.
I have nothing to back this up, other than the fact that the author said Muv Luv Alternative was an inspiration. What if he didn't just get the grimdark hopeless aspect of it, but also got the idea of a protagonist going through the motions over and over again?
  • Well, the very beginning of the manga (and subsequently anime) does provide sufficient bait for this, as Eren wakes up with the feeling of having had a long dream (the end of which is Mikasa saying "see you again, Eren"), tells Mikasa "your hair grew longer, didn't it?" (when in the 'future'/life or lives he'd lived previously, it'd be short), and crying for no reason (or so they think)... Maybe Eren being incredibly adamant right afterward that the soldiers be ready "WHEN THEY BREAK THE WALL!" comes from residual memories of that previous life or lives he's had to survive, and must come to survive again.

Somewhere out there, there's a Reset Button, and Eren's been the one pushing it.
Going with the above theory, Eren wakes up in the first chapter the way he does (crying, wondering about Mikasa's hair length, etc) because he's just coming around into the new start of a Stable Time Loop, but can't hold on to those memories and is stuck going through the same loop. That's what's in the Jaeger house basement: not just the secret of Eren's turning into a Titan, but Grisha's collected knowledge of the loop: the nightmare Eren had about his father acting weird since his mom died and doing creepy science to him happened in a past loop, the same way his opening dream did. Grisha's become aware of his son's ability to carry himself through time, and has been gathering new resources for his son to save humanity every time Eren starts the loop over.

Instead of Time Travel or a Reset Button, there are SEVERAL massive Memory Gambits.
Alright, there are some inconsistencies in the timeline, and several people, even some in-universe(specifically I'm talking about Erwin and his father) have theorized that the memories of humanity have been altered, and the series has a minor theme about memories- especially inconsistent ones or ones that actually happened differently from how you remembered them, and there are some times where certain characters(Sasha, Armin, Mikasa, etc.) would remember something but not be sure why they remembered it in the first place. So, wouldn't it make more sense and fit in better with the series(meta-wise and in-universe) if the examples are caused by altered memories rather than time travel? Perhaps even Geographia/The Woman or someone connected to her(ancestor/previous co-ordinate, maybe?) was the one who started all the memory alteration and the mind tricks.

Carla knew more than she let on.
Going with the above two theories: when Hannes shows up to rescue Carla, Eren, and Mikasa in the first episode, he first goes to face the approach the Titan, Carla says, "You can't fight it.", not "forget the Titan, save my kids". It could just be that she had no faith in the military (which is why she's so against the idea of Eren enlisting), but she's crushed under a building and can't actually see the Titan until it picks her out of the rubble; she would have had no way to gauge whether Hannes was up to the task. If there really is time travel or a reset button at work, Carla may have been aware of it from a previous iteration.

Titans are essentially an organic alternative to Humongous Mecha, and their concept was based on the Evangelions.
Mecha created with organic parts, piloted through a neural interface in the nape of the neck, and can occasionally go berserk against the will of its pilot if not controlled enough. The Titan Shifters are also, like Evangelion pilots, the only hope against the abominations that are invading the city (Titans/Angels). If the "Titans are Lotus Eater Machines" theory is true, that makes their concept as Eva-lites even more apparent because in the end, the Evangelions were the ones who eventually turn the world into a giant Lotus Eater Machine during Instrumentality

The Walled City is a Genius Loci, the Titans are The Corruption, and there is no outside world.
It's held in place by the dreams of the Titans in the Wall. As long as the Walls stand, exposure to something in the outside world can't mutate normal humans into normal Titans, but the Walls are gradually coming down and allowing the Titans in, and they're carriers of whatever changed them. The world beyond the Walled City was destroyed long ago, and everything the characters can see of it is just the Wall Titans' memories made manifest.

The Wall Cult is the real villain
As unlikely as it is, perhaps Reiner, Bertholdt and Annie know something that humans have kept secret from Eren. While the majority of humanity is blissfully unaware, the Wall Cult and the corruption within the Military Police shows that there is more to everything than face value. While King Kong (bad pun, I know) is causing trouble, Reiner, Bertholdt, and Annie were trying to save Eren, a fellow Titan Shifter, before the Wall Cult could figure out a way to execute him or before Mr. Sasquatch could kill him — as it seems that he is unaware of the Armored, Colossal, and Female Titans if he was unaware of the military tech of the time while the group had prior knowledge. The Wall Cult has been trying to control humanity, and while the Titans are indiscriminately killing, they are perhaps trying to take down the Wall Cult who have subjugated the colossal Wall Titans.

The Walls were built in a compromise between humans and Titans, and the Wall Cult is tasked with keeping that knowledge from the public.
Continued from the above WMG (I promise I'll stop after this): The complacency of humans is a big thing in this series, along with the livestock metaphor. Humanity had already been mostly wiped out by the time they made it to the Walled City, and the Walls had been prepared in advance. The human leaders who made it happen were basically making a deal with the devil: the Titans would provide humankind with a safe, secure place to live and assured survival of the species, and the humans would provide Titans with sacrifices. That's the Awful Truth that the Wall Cult is hiding; the peace of the Walled City is literally paid for in blood, but when that arrangement was made, it was the best way to ensure that the human race wouldn't go extinct. Historia's family was give the authority to release the truth in the event that someone finally finds a way to wipe Titankind off the face of the earth for good.
  • .....suddenly the entire purpose of the Scouting Legion becomes more horrifying. If this is true, then they are the sacrifices and also serve as a method of removing people that are too curious, while creating a deterrent for everyone else. It would also raise more questions about the motivations behind the attack on humanity, since that would mean the Wallbreakers were breaking the agreement. Radical factions opposed to the deal, maybe?
    • I don't think so; if the Wall-breakers were opposed to the Walled City/humanity's existence, there wouldn't be any reason to infiltrate the military, they could have just kept up the assault on the Walls until the Titans swarmed the center. Bertholdt mentions that his plan was to join the Military Police, so maybe they're after something specific in the capital? Either way, yeah, the Scouts just got pretty damn creepy, and it means that the government has been using military operations as an excuse/euphemism for sending people to the slaughter. If all this actually does pan out? Armin's grandfather was probably exiled because he owned that book Armin treasures so much, and Grisha may not have been far behind him. The innocence/ignorance of the Walled City is that crucial to the upper class and the Wall Cult.
    • Wall breakers aren't opposed to humanity. They're opposed to the Walls. Feeding off the human livestock theory, imagine where Berthold and Reiner's village was. That's right. They got hit first. Now what if they discovered their villages were left so far on the outside to be sacrifices, while the corrupt inner Walled Cities got to live nice and comfy on their families and friends' blood. Now, breaking the Wall isn't so much of an act to exterminate humanity, as an act of putting everybody in the same frying pan. So why infiltrate the military? Why not keep up the assault? Because they're not natural Titans and the the natural ones would attack them too. It's known now that Titan Shifters get tired in their Titan forms as well. The best option for them is to break the Walls, return to human forms and travel to the next with the other humans, allowing them safe, rested passage right up to their next target. Now, starting from the inner most Wall and then moving outward is a different story. Their only opposition would be the soldiers(lol at that), which would make Berthold wanting to be MP make so much sense. So what went wrong? Reiner Went Mad From the Guilt and had a personality split. The Soldier who wanted to become a scout, protect his team, and defend against the Titans — and the Warrior who set out to destroy the Walls. The Soldier in Reiner deviated from the plan and became a scout. Berthold(who never realized the split until later and began triggering switches by mentioning soldier/warrior) went along with it, because he's not only rather weak willed (as said in his aptitude test) but Reiner's best friend.
    • Do we know they're not natural Titans? It seems like the regular Titans just eat whatever they think is food, and Titan Shifters still ping as tasty humans to them. They also seem to know what's up with Christa and have reasons to want her kept safe; possibly what they wanted in becoming MP's was her, or someone in her family line, to get to the bottom of the Titan problem. They don't seem to have been in a different boat than humans were, what with a childhood friend being eaten and all, they were just dealing with Sasquatch Titans and Ymir's People.
  • A separate theory for why the Wall-breakers would've gone against the agreement: Erwin Smith is considered to be an amazing commander because he invented a strategy that significantly dropped fatalities in the Survey Corp. Maybe whoever leads the Titans ordered the Wall attacked because they weren't getting all the sacrifices they were promised? No idea why discovering Eren would make them change their plans, tho...
    • Because the people the Shifters work for are separate from those who agreed to make the Wall for humanity in exchange for sacrifices. What if the group who wanted sacrifices in order to create Titans are Beast Titans, who are capable of turning people into Titans? Reiner's people are a group of people who were excluded from the nobility who first populated the Walled City, and somehow developed a means of giving people the power to turn into Titans as their only way of fighting back. The Walled City not only left them outside its safety, it also increased the number of Titans they had to fight due to the deal with the Ape Titans! It was after the city was attacked that the Beast Titan chose to came in and intervene—in order to preserve the steady supply of sacrifices. Perhaps what they use to create Shifters comes from the Beast Titan, which is why Reiner and co are after it, and also why they'll settle for Eren: He's not one of their kin, so he may be a potential source of a manufacturable shifting formula—the only weapon capable of giving them a fighting chance. It would also make Ymir's comment of having "stolen the Titan's power" from them make a lot more sense too.

The giant trees are artificial.
They were genetically engineered to compensate for the massive damage done to the climate by the wars.
  • This is unlikely, as trees such as these actually exist and are called Sequoia trees. Whether or not the trees depicted are Sequoia Redwoods is unknown, but if they are it would at least imply they were artificially planted since Sequoia are only found in one part of Earth.

The Walls aren't the only structures in the city that are filled with Titans.
Chapter 9 shows a Titan, apparently (and awkwardly) gnawing on a support pillar in what is implied to be one of the oldest buildings in the city. This is the same chapter that shows Titan Eren being eaten by other Titans, apparently being able to sense the tasty human meats inside him; maybe ALL the major architectural structures use Titan foundations. They do come in all shapes and sizes, after all...

The regular Titans are deformed because whoever is changing them is getting sloppy with their work.
All the Titans are friggin' horrible anyway, but the further along the story goes, the Titans seem to get more and more deformed; the Sasquatch Titan in particular crushes one that has such a messed up face it looks like something out of Picasso's nightmares, and the one that is most likely Conny's mother can't even move itself because its limbs are too small. It may be because changing humans into Titans is supposed to be a delicate, careful process, but whoever's responsible for performing the "operation" is getting desperate and cutting corners.

The name is not gramatically incorrect in English
It takes place on Saturn's Largest Moon.
  • That would explain the runes. The ships in question could be spaceships, and the "continent" could be Titan itself. The characters are the descendants of people who fled Earth for Titan, and most of their technology was destroyed in a war that occurred soon after they landed.

Annie told Marco everything before she killed him.
I don't have a lot to base this on, except Marco looked peaceful and calm after he died. He may not have been a top-notch hand to hand combatant, but if Annie killed him suddenly or violently he would have been panicking and ended up looking more or less like the other Titan victims. She told him everything, knowing that it'd become a "Now I have to kill you" scenario... but Marco, being thoughtful and unusually good at seeing the best in people, understood, and didn't resist when Annie finally did kill him.

Marco was killed by the Wall Cult, and they gave Annie his gear.
Something's been bugging me about Marco's death. Yeah, Annie took his 3DMG and the evidence points to her at least being the last person who saw him alive, but look a little closer at his body when they find it: he's been smashed through a fence, which has tucked (what's left of) his chin against his chest and probably broken his neck, but his hand is still making a fist and he's flat on his back. Now, imagine how he would have looked if a Titan had picked him up and bitten off his upper right side, then tossed him back on the ground: he'd just be flopped down like a rag doll, right? But he's not. It's because he'd been dead for some time and had already gone into rigor mortis when the Titan bit him. He had to have been laying flat on his back with his hands clasped together on his chest for him to land in that position! In other words, whoever killed him couldn't bury him, but still posed his body according to religious rites for the dead, something that the Titan Shifters would probably have no idea how to do, much less be inclined to do it at all. Considering that the Shifters all know who Christa is and why she's important, and so does the Wall Cult, there's no reason that they couldn't all be part of the same conspiracy.
  • I really like this theory, though admittedly it's mostly because I don't want Annie to have killed Marco herself.

There will be more references/shoutouts to Mythological Giants
Assuming there are more Titan Shifters out there, and that they are probably going to be some unique things associated with them, there will be more characters and other stuff that will reference Mythological Giants. We already have Ymir and Utgard which reference Norse Mythology, and the Titans themselves, while some would argue is wrong, reference the pop culture view of the Titans from Classical Mythology.
  • One of the Norse myths was that Loki made a deal with a mountain giant to construct a wall to protect Asgard, but the giant was tricked at the very end and pretty much built the wall and received nothing in return. It might shine some light on the origin of the Walls and why humanity is being attacked(?)

Somehow, we'll have a Thor and Mjolnir reference
Considering Norse Mythology is a recurring source, it's likely.

There's a massive Memory Gambit in the works.
And its possibly all because of Eren's dad, or at least the serum involved in Titan Shifting. In Chapter 46, Reiner also reveals that his memories are pretty faulty. This could mean its only for the people who became Titan Shifters through the serum, but throughout the chapters, there have been instances where many characters go into flashbacks wondering why they're remembering those memories in the first place. So far this includes Armin, Mikasa and Sasha. As a well regarded doctor, I don't find it too farfetched that Grisha could've given his patients something that could cause them lapses in memories. And its more than likely he was involved in treating people who suffered from the plagues in Sasha's village.
  • Reiner's memory issues are his own doing, according to Ymir. But I definitely think it's possible that Dr. Yaeger did something to his various patients when he cured that plague. Memory seems to be a significant thing in the series, especially in terms of it being unreliable or spotty.
    • In the anime, at least, Eren's flashback to Grisha injecting him with Titan-juice has him talking about how he knows Eren won't remember it, but they're going to have to gamble on "their" memories. Maybe he meant it literally?

The Titan that nearly swallowed Armin and actually did swallow Eren was Armin's grandfather.
Because now we know they're people, and Armin's grandfather was sent out to reclaim Trost and never returned. We never really get a good look at him, but it makes sense that a balding old man would be attached to his hat, and a smart guy like he probably was could probably get by pretty well out there in the wilderness for a little while. Not a very long time, mind, but maybe long enough to grow a beard out... It's also notable that although he picks up Armin and tries to eat him, he doesn't try to eat him again once he escapes, even though he was right there in front of him, traumatized and rendered a helpless sitting duck.
  • Chapter 10 of the manga shows that the Titan didn't eat Armin because Eren burst out of him before he could (and I guess Armin was too traumatized to remember that part). I still like this theory though, since it maximizes potential angst.
    • Not quite; between the moment it swallows Eren, and Eren finally transforms into a Titan after he despairs, it spends a panel just staring at Armin. More possible evidence: knowing what we do now about Titans, there's a pretty good chance at least some of them were people sent outside the Walls...

The Female Titan is Dot Pixis' bane.
What he said, that he wouldn't mind being eaten by a truly beautiful Titan woman, was a case of Foreshadowing. If/when Annie is broken out of her crystal, she will kill him.

Perhaps the insides of the Titan necks where the pilot is hinted to be traps the pilot in a Lotus Eater Machine
Perhaps the pilot wakes up in a perfect world where everything they experienced was a dream. It's already hinted that Titans are really humans, so why not. Eren seemed to be in one anyways, where Armin had to talk some sense back into him to pick up that boulder and fix Wall Rose.
  • Ymir may have confirmed this, at least in that the person is trapped in a dream-like state. Her experience was described as a nightmare, as opposed to Eren's peaceful dream. Bertolt states it was "the same for us", suggesting that the only difference between normal Titans and the Shifters is that the later managed to wake up eventually and return to their human forms.

The settlers weren't the only humans left, and they screwed over the people outside of the Wall.
So we know that humanity is responsible for nearly wiping itself out, and then that the Titans came along at the worst possible time. The few surviving humans, when escaping the Titans, came across the Walled City and found asylum in there. So here's what I think: the settlers weren't the only surviving humans. There was another group fighting the Beast Titan, who could create Titans from people. By studying it, they created Titan Shifters. This is what allowed them to create enough Colossal Titans to create a safe haven. The settlers came in later, and were unable to fight as Titan Shifters, most likely because the method of creating Titan Shifters is imperfect and resulted in the first Titans we saw, which surrounded the city and could eventually become Shifters. So the settlers, who were all selfish nobles, screwed over the Titan Shifters and sealed them out. Ages passed until the Beast Titan somehow got in, and the outsiders were forced to risk everything by sending special Shifters into the city. However, around the same time, the genius Doctor Jaeger happened to come across evidence of the past and perfect the Titan shifting formula, giving it to Eren. This is why Reiner's group is willing to abandon their efforts to wipe out humanity and/or capture the Beast Titan for Eren: they know that there's a chance that he's got the secret to the perfect Titan shifting formula in him, and can finally end the war.

Once they find the Jeager household, several members of the cast will become new Titan Shifters.
Whether it be some accidental explosion/spill (I should think that if you smash a house with a lab under it, things may have been broken inside, or scavenging around may trigger an explosion/spill), or they voluntarily decide to use it to gain an advantage (admit it — new Titans may be awesome), the possibility of more Titan Shifters, be they from the cast, or someone recovered from the Titans, does exist.

Abnormal-class Titans are early bloomers, figuratively speaking.
We know the Titans are humans, trapped in a dreamlike/nightmare state, but that after a very long time, are capable of coming back to themselves in the form of Titan Shifters. But all the ones we've seen so far are classified as abnormal types rather than by their height: Reiner has armor plating, Bertholdt is huge, Annie's a babe, Ymir's a Dance Battler, Eren is some kind of violence elf. And there are a few abnormals running around in the show (that flying-leap one that eats Thomas, for instance) that have unusual capabilities; it could be that all the Titans will eventually come back to themselves, it's just a matter of time, but the abnormal ones just reach that stage earlier than typical. Ymir's the closest of the known Shifters to a normal Titan, and we see from the flashback that Reiner and Bertholdt had already regained their human forms as of sixty years ago when Ymir regained hers. It could be that the 100 years that Titans have been running around simply haven't had enough time, but they inevitably will. Which might mean that...
  • You've got some details wrong. Ymir was a normal mindless Titan for 60 years, she hasn't been a Shifter for 60 years. She states that she was only able to recover her human form 5 years prior.

Erwin is a bad guy

Humans really did build the Walls.
The closing credits have a bunch of images painted on the Wall that divides the cast, including ships sailing across the ocean to reach the Walls, opposing groups on horseback apparently marching on each other, and a paper-doll-chain of huge figures that could easily represent the Colossal Titans that make up the core of the Walls. Among those images is a bunch of men on horseback, dragging a pallet with what looks like a Titan strapped to it by its wrists and neck...

The lighting from Titan shifting are transmutation sparks.
They transmute the surrounding material to create their bodies. The injection Eren received gives the ability to perform this specialized alchemy.

Titans eat people because that's how they change back into humans.
Going with the theory about the Walls being not a refuge but a breeding pen: Ymir says she "stole the Titan's power" from a childhood friend of Reiner and Bertholdt. If that kid was a normal human and not a Shifter (and since he didn't Shift to save himself, that may have been the case) it could be that something about the act of eating a human has a chance of triggering the change. They really do eat to survive, just not in a way that makes sense to humans.
  • Overall a good theory, though one nitpick. Ymir states she stole the Titan's power "a long time ago" and references her crime as her motivation to seek out Krista. Since she didn't know about killing Berrik at the time, she's probably referring to how she became a Titan in the first place. But Berrik's death and Ymir's subsequent return to human form suggests there's a connection between eating people and becoming human again.
    • I think that depends on how long "a long time ago" is, but Ymir does seem to be the odd man out among the Shifters so far. But, going on from there...
  • Partially confirmed. Hange theorizes that Titans instinctively eat humans because they have a Cannibalism Superpower and eating a Shifter makes them one.

"Offering up your heart for humanity" is more literal than it sounds.
We see a lot of people get bitten in half, and corpses with missing limbs and missing limbs without corpses... but never any organs. It could just be because that's a little too much gore for a mainstream manga, but what if they don't eat humans as much as they eat human hearts, and eating the right one, or enough of them, is somehow what turns them back? It would also lend a bit more meaning to the scene where Mikasa knows Eren because she can hear his heart beating...

She views herself as inadequate and worthless, depends strongly on other people to guide her life, puts on a kind and thoughtful mask so others will view her in a positive light... It all fits.

Humanity's leaders betrayed the Titan Shifters.
Building somewhat on one of the other WMGs around here somewhere, with humanity warring against the Sasquatches before the Walls were built, the Shifters were probably created as weapons. But when the war ended in a tenuous peace treaty, the humans agreed to destroy most of them, probably with one of the leading Shifters talking a lot about self-sacrifice but secretly keeping himself alive and living in luxury, or something similar for the king or whoever. This left some of the other survivors rather jaded about the concept of self-sacrifice and doing things for others. Also I'm pretty sure the Wall Titans are actually ape-types, just big ones. They resemble Sasquatch a lot more than any of the others if you look closely. Then again, they might just be a type we haven't seen before.

The series will have a Book Ends ending
It'll end with Eren sending his consciousness back in time to the day the Colossal Titan arrived.
  • Alternatively, it opens up with Eren waking up similarly to how he did at the start of the series, but this time he's outside the Walls, the war has been won, and his tears are of joy.

Four Factions theory
This is a theory based on some of what we've seen so far, detailing the possible entities in the world.

Humans, or "Soldiers" This faction consists of the Walls, including the royalty. Having originally lost a war with the Titans, despite developing Titan Shifters, they formed a peace treaty with the Titans, betraying their Shifter weapons, to be imprisoned within the Walls in return for peace. Their goal is to maintain the status quo, either permanently or until the Titans can be defeated.

Titans, or "Them" These are, of course, the Titans, led by high-ranking officers such as the Beast Titan, and presumably others of his kind. The actual leaders of this faction are currently unknown. During the original war, the Beasts, possibly an invading alien race, being a species with the unique physiology of a small vital pilot in a much larger but regenerating body, created an army of similar creatures with humans as the basis in order to ensure victory. They agreed to the peace treaty, allowing some humans to remain alive in a limited capacity to use for their unknown goals, whether that be breeding them as food or experiments of some sort in order to perhaps restore their dying race.

Shifters, or "Warriors" The Shifters are the descendants (or perhaps still the originals) of the weaponized Titan Shifters developed and deployed during the war to counter either the Beasts or their Titan minions. (If the former, the Titans may in fact have been created from a corruption of Shifter technology, which would itself be based on Ape physiology.) When the peace treaty was written, they were exiled or killed, but at least some of them survived. They have a tendency to be extremely jaded and cynical. In 845, they attacked the Walls in order to force humanity to fight once again, but abandoned this plan in 850 with the discovery of Eren Jaeger, who they hope will allow them to create a new army of Shifters to help defeat the Titan faction. It's likely that the methods used to create the Shifters to begin with were destroyed after the treaty was signed, but kept/rediscovered by Grisha Jaeger and used to change Eren.

Liberated Titans, or "Ymir's People" These people are Titans who have, by unknown means, regained human intelligence, becoming beings similar to Titan Shifters, but with more typical Titan forms. Trainee Corps member Ymir belongs to this group, and could be its founder, and the one with knowledge of the methods used to restore Titans to human status. They appear to be currently in hiding, with no particular ambitions other than survival. It's possible that their ranks consist solely of former Aberrants, with rank-and-file Titans being irretrievable, as Ymir's Titan form strongly resembles an Aberrant rather than a normal Titan.

A Coordinate built the Walls
Once, long long ago, a coordinate was somehow found/born/reawakened by the humans, and this Coordinate also joined the human side. Eren can command Titans, right? So what if this Coordinate somehow commanded thousands of Colossal Titans to form the Walls? Now isn't it obvious why Bertholt thought that power was too strong for Eren to handle?
  • Then this person had kids, who formed the church-nobility family Historia comes from. Reiner thought Historia could be useful, so unless she has some really valuable information they don't have, it's possible that they want her for her genetics. People have been suggesting Titan-shifting is genetically passed down. Well, what if that's true for the Coordinate, but the genes are recessive? Or partially recessive? Historia might not be a Coordinate, but she might have the potential to become one under certain circumstances, or to give birth to one.
  • Alternatively, three members of the Reiss family named Sina, Rose and Maria had the Coordinate power, built the walls and if the Reiss family gets their way, Wall Maria is about to become Wall Historia.
  • The Coordinate also tinkered with the minds of the people.

Titans have always been around
Assuming the show does, in fact, take place on Earth, then they share our history. That means that there were many cultures with giants in their mythology and folklore. Maybe the Titans had always been around, but sleeping, until humanity did something stupid and woke them all up. Maybe mixing a little science with magic to try and make war machines.

Titans can't see color.
Not even gradations of color; their vision isn't analogous to human sight at all.
  • When Annie is looking for Eren, she needs to look at Armin's face to identify him — even though it should be immediately obvious, since Armin is bright blonde, while Eren's hair is black. In fact, Titan vision in general is probably very poor, since there's a number of indicators that should have told her Armin wasn't Eren without needing to check, and she definitely shouldn't have been confused between Reiner, Jean, and Armin, none of whom are built remotely like each other, let alone like Eren.

There is a Necromancer Titan.
So we have the Beast Titan who is all but stated at being able to turn living humans into Titans. So the logical extreme is one that repurposes the restless dead into new Titans. Possibly grafting the dead souls into new bodies using its own flesh so it always appears as a starved, near skeletal like corpse. Alive or dead the Titans will screw you over.

As of Chapter 51, any Plot Armor previously owned by the characters are now gone.
Attack on Titan has never been merciful to its characters, but now, with Jean, Mikasa, Armin, Sasha, Historia, and Connie directly in charge of Eren's safety, any hint of immunity to death they once had is now gone. As of Chapter 51, all of their backstories and motives have been revealed, making them all ripe for the killing. God speed.

Hard Steel is Titan Skin
The blades used with the 3DMG are noted as being incredibly light, and the only things capable of tearing Titan flesh without breaking. What else is described as being surprisingly light, and noted as being able to kill Titans? Other Titans. Titans have been shown to be capable of producing particularly hard skin, as evidenced by the Female Titan and the Wall Titans, and Hange gets a piece of the stuff which fails to disintegrate like normal Titan flesh, so it seems logical that people would use the stuff as a material for the swords.

Also, the manufacturing process for creating hardened steel is said to be a trade secret, and the people running it probably wouldn't want the public to know that it's Titan skin.
  • Those smokestacks shown on the visuals for Hardened Steel production? It's not Smoke. It's steam.

The 3DMG runs on Titan blood vapor.
Expanding on the theory above, it seems a bit odd that gas compression would be possible in the setting, given that the level of technology is clearly close to that of the Renaissance, and gas compression wasn't widely implemented until the twentieth century or so.

However, Titan blood has been shown to evaporate very, very quickly, and the vapor from it looks suspiciously similar to the gas that comes out of the 3DMG. Furthermore, the 3DMG gas also dispels very quickly upon being expelled. And Titan blood, wouldn't be hard to bottle up and distribute, given that it would do the compression by itself upon evaporation. The soldiers wouldn't suspect a thing, given that opening a canister of "compressed gas" would be sensibly dangerous to them and that Titan matter is very light.

Combine this and the hardened steel with the conspiracy theory that the government is actively trying to maintain a breeding pen with the Walls, and suddenly things start making sense.

The reason Titans are single purposed
Is because they are controlled by human brains. Most large animals have larger brains to maintain precision over movement and sensation. The human brain doesn't contain enough nerves to control and take in all perception from the Titan body. To get around this, only parts of the Titan's body are connect to the nervous system and the brain loses its general intelligence. This also explains why some Titans feel pain but others don't.

Levi is secretly a woman
Because that would be the ultimate troll move towards fans and Isayama is fully capable of it.

The Titans self-destruct upon death
When the Titans are killed, they seem to evaporate. Parts of their bodies that are removed do the same, even the smallest drops of blood. It also destroys the bodies of the people they have eaten. It seems likely that rather than burning up from the heat that they self-destruct producing a large amount of heat. The Colossal Titan can control this to destroy his body very quickly.

The next Titans we see will be able to counter 3DMG
Since the Beast Titan took the 3DMG, it will probably alter new Titans to be able to nullify the at least standard tactics.

The faction within the Walls.
They're talked about a lot in Chapter 52. All we know is that they are frantically trying to track down Eren and Christa and murdered Pastor Nick after he wouldn't give in to their torture, and that they're connected to the military police and royal family. So, who are they? What do they really want?

Historia is a Child by Rape.
Historia noted that her existence was painful to her family, and that her mother would go out every night to meet someone, earning income from an unknown source. She treated her daughter as something horrible, that she could not stand to associate with. The way Lord Reiss tossed her aside suggests he never cared about her to begin with, as he shows more compassion towards the daughter he's literally just met. As such, it's possible the relationship between Historia's parents was not consensual, but rather a nobleman forcing an attractive servant into sleeping with him.

Historia's mother was a prostitute.
She's a beautiful farm girl who dresses up in fancy clothes and gets a ride to the city in a carriage to go and make money, with an illegitimate daughter but no apparent lover in her life. Given the nature of the setting, I don't think she was going there to sell insurance.

Pastor Nick did break under torture.
In Chapter 52, Pastor Nick has been removed from the Survey Corps' care and seems to have withstood torture by nail-removal (i.e. all his nails were removed whereas the removal of just one would be sufficient for a normal person to crack) and Levi praises him on his strong will. That is just conjecture on Levi's part however, as his torturers could have just as easily been sadists who continued even after they got the information they wanted, or they did it after they killed him to cover their tracks.

There is a fourth Wall keeping the Titans in
The country that everything takes place in is actually a massive prison for the Titans, with humans stuck in the middle to serve as bait to keep them from breaking out. But after a hundred years of nothing to eat, they started attacking the fourth Wall. Annie, Reiner, and Bertolt came from the lands outside to break the inner Walls to cause the Titans to press inwards, away from the outer Walls. This would mean that much of the rest of the world is intact and safe, and would explain why Annie, Bertolt and Reiner continue to act on their mission despite being so traumatized by it, they're just trying to protect THEIR homes by sacrificing others! The Wall Cult could serve as Wardens in this sense, fully aware of the truth of their country, but complicit for the sake of the rest of the world, using religious dogma to motivate people to protect the Walls when the real Wall they care about is the one no one knows exists.

There's a limit as to how much Titan Shifters can shift
Just as it says. Maybe once they've exceeded their limit, their bodies become too engrossed to their Titan forms that they just practically disappear. Which explains as to why the army couldn't find any human inside the nape of Regular Titans. The limit also varies from each individual. Also supports the theory that all regular Titans were once humans.
  • Seems to be partially confirmed. After hours spent in Titan form and multiple transformations, Eren's body becomes partially fused to his Titan form.

Annie will teach Eren how to use the hardening ability and possibly more
When Annie is released from the crystal she will ofter the chance to live by teaching Eren what she knows about the Titan's powers including the hardening ability that she has made full use of.

The Reiss family and the Wall Cult are responsible for sacrificing humans to try and appease the Titans.
The book Historia was reading in Chapter 54 shows an image of a girl, holding an object which looks like an apple (but could also be a heart) to a Titan with some resemblance to Ymir. The Reiss family and the Wall Cult are intrinsically connected, and the Wall Cult is somehow responsible for placing giant Titans inside the Walls. It's possible that other methods may have been use to protect humanity, including sacrificing specially picked young women. Which leads on to another theory´┐Ż

The girl shown in the book in Chapter 54 is Ymir.
Hair colour aside, Ymir mentions in an earlier chapter that she died for the happiness of a great many people, which suggests some kind of sacrifice. The book seems to show a young girl being sacrificed to a Titan who looks similar to Ymir's Titan form. If the theory of Titans somehow regaining humanity by eating people is true, Ymir may have become one with the Titan who ate her and returned.

Nazis are involved? Some other form of eugenics?
There are hints that SNK happens in a world related to ours. What if it occurs in some alternative universe, and it occurs after world war 2 (and not 3). It was mentioned explicitly that the concept of race is unknown to many people (see the talk between young Mikasa's kidnappers). Furthermore, ote the frequence occurence of german names. The SNK Walled City is some kind of an eugenic experiment designed to create a single race of human beings. A single race of human beings may also be the "solution to human wars" which was mentioned previously.

The Woman is Historia's sister
Based on her apparent resemblance to Historia and her father, the woman is the legitimate daughter of Reiss and a member of the Wall Cult. Her memory powers are a means for the conspiracy to control the population. Grisha also has ties to the Wall Cult and conspiracy, and transferred her memories to Eren. She was teaching her half sister how to read out of kindness and erasing her memories to prevent her from becoming involved in the cult. She was so horrified by Historia's desire to be like her because she knows how bad the cultists truly are, and worked with Grisha to undermine them.
  • Confirmed. Her name is Frida Reiss, Historia's half-sister.

The Woman was the one from Eren's dream at the start of the manga
She's the one who gave him the scarf, which he then passed on to Mikasa, which is why he mistook the woman in the dream for her. This is important because it means that Grisha had already been screwing with his memories before the Colossal Titan attacked.
  • He mistook the woman for Historia, not Mikasa.
    • That doesn't mean that unclear memories + familiar object ≠ erroneous interpretation
      • I think he was talking about Eren's dream from the beginning of the manga.

The Titans aren't just keeping the humans in.
They're keeping the Shifters out.

The history of Humanity is a lie.
Erwin states in ch. 55 that his father believed the government altered the memories of the people 107 years ago, around the exact time Titans supposedly appeared. This was done to control them (using the Coordinate), and we learn from Djel Sanes that the Secret Police actively eliminate intellectuals, explorers, inventors, and anyone who gets too curious or rocks the boat in any way. We see that one inventor killed was building a revolver, fitting with prior hints at the government preventing technological advances. The soldiers are trained in various skills, but it is noted early on that things like self-defense are not prioritized and most simply go through the motions. The government does not want a population with the tools or skills necessary to fight other humans, and the Survey Corps is their tool to help maintain the status quo in two ways: it eliminates the freethinking and curious via Titans, while also providing a constant reminder to the citizens that the outside world is bad. The Walls and the Titans exist to keep humanity in, to prevent the people of the Walled cities from attempting to leave. Eren's comparison of people to livestock is accurate, in that humanity is being kept in confinement and encouraged to not think for themselves or dream of leaving. Annie and Reiner both actively encourage not only honing fighting skills, but freethinking that questions the status quo because they are aware of the government's efforts to keep the population domesticated.

There will be a pseudo industrial revolution following the conclusion of the revolution arc.
The reason there have been no major innovations for so long is because any and all intellectual thinkers and inventors who weren't completely dedicated to the royal family were assassinated. Who's to say they didn't keep some of their research and the like for themselves though? And who's to say that this stuff won't get out once the Central MP is out of the picture? Or that people who realized what was going on and chose to hide their work as a result won't come out of hiding?

Marlowe will prove to be a Chekhov's Gunman.
Many seemingly minor characters have since reappeared and played unexpected roles in the story. Marlowe is one of the few soldiers from the Military Police to get any development, and he explicitly wants to change the system for the sake of the people. He will reappear during the revolution arc, possibly aiding in the efforts to overthrow the corrupt government. Hitch, the other MP given development, may also play a role in the upcoming revolution.

A Shifter's powers are activated by emotions.
The Self Harm and pain triggers a fight or flight response, which combined with a conscious goal, creates the Titan body as a survival mechanism. The shrieks Annie and Reiner released were the result of fear of being captured...and possibly self-hatred since the Titans Annie called tried to eat her. Annie's crystal prison was created by sadness, combined with some symbolism in her Single Tear (blood=muscle, energy; tears=salt,clear crystal). Eren's Volcanic Veins and resulting strength were caused by rage.

The couple getting into the hot air balloon while the Military Police opened fire on them in Chapter 55 were Armin's parents
Armin's parents wanted to go beyond the Walls and clearly never came back. What better way to get beyond the Walls with the technology available to civilians than building a hot air balloon to fly over them?
  • It's a big possibility in the anime, which adds a special line from Armin in Episode 5 that his parents were sneaking outside the Walls while he and Eren were children. In the manga, his parents were vaguely killed thanks to the military, presumably as part of the suicide Wall Maria expedition, but the possibility still exists.

It's not just the three Walls that are made out of Titans
Whenever we get an oversight of the Walls, it's not just the Walls themselves that get highlighted, but also the castle within the inner Walls. Stands to reason that that castle is part of the same system. In addition, in one story, a digger tries to go under the Wall, only to find the Wall going down all the way to the bedrock, which seemed to be made from the same material. Now that the Walls have turned out to be made from Titans, isn't it possible, if not probable, that that bedrock is also made from Titans?

The memory alteration removed the language barrier
In Castle Utgard, there was a can of food with a language only Ymir could read. Indeed, the others didn't even seem to recognize the alphabet. However, if you look at the can, it appears to be german writing, with a z and a ▀ visible. Considering how prevalent german naming is throughout the series though, it seems odd that the alphabet wouldn't survive. However, it's been speculated by Erwin and his father that humanity has undergone a memory alteration when they moved into the Walls. Given the survival necessity of a unified society within the Walls, it's possible that whoever altered everyone's memories chose to teach them all the same language and remove the memory of all others, creating a society without language barriers.

Levi is a member of Mikasa's family
Be it a long lost brother or a cousin, it makes sense that the two of them are related, given their similar appearances and their skills. While Mikasa takes more after her mother, Levi would have more of the Ackerman blood in him.
  • We know that he at least has the surname Ackerman, though their exact connection remains unknown. However, it's unlikely for them to be siblings based on the large age difference compared with the apparent age of her parents.

Mikasa's last name is meaningful, just not for her
It's been pointed out that everyone in Attak on Titan has a meaningful last name. Everyone but Mikasa, since Ackerman means farmer and she doesn't farm.

Come Chapter 56, we learn she shares a last name with Levi and a man who's only referred to as "Captain Ackerman." Captain Ackermans is apparently part of a human-suppression force. Since Levi's last name wasn't revealed until this chapter, it's likely that the connection is significant to the plot. Most likely, Mikasa, Levi, and that man are all related — thus, explaining the same last name. The fact that a seemingly human group calls themselves the human suppression group, it's likely that this group isn't made up of humans. So Captain Ackerman isn't human. Since we already know that the last name "Ackerman" is relevant to the plot, it's possible that there will be other non-humans with the last name "Ackerman" who show up, possibly as part of the human suppression force.

What's the most obvious trait Mikasa and Levi share? They have amazing fighting skills. If one guy named Ackerman isn't human, perhaps Mikasa and Levi aren't human either- and that could be why they're so insanely powerful. If Isayama makes the last name Ackerman really plot relevant, it could even be that everyone who's a part of this particular family (barring people like Mikasa's mother who married into the last name "Ackerman") isn't human.

Eren makes a lot of comments at the beginning of the manga/anime that human beings are no better then animals to be slaughtered by the Titans.

Ackerman is a meaningful name, for any Ackermans in the human suppression force. If humans are animals, Ackermans are the farmers that watch over these animals until slaughter time.

The Ackermans are anti-Titan Shifter weapons who serve Coordinates
The two Ackermans we've seen in combat so far, Levi and Mikasa, have also been the only people capable of effectively fighting a Titan Shifter (aside from another Shifter). This is because they've been modified by the conspiracy to be stronger and faster than normal humans to be more effective against Titans, specifically the Shifters, who are working to destroy the Walls. We know that Captain Ackerman, at least, serves the conspiracy, so obedience to those with Coordinate powers may be another part of their design. The reason Mikasa is so protective of Eren is because she's instinctively aware of what he is, and is compelled to protect him.

Also, the Shifters are aware of what they are. When Mikasa asked Annie to teach her some moves in a flashback, Annie responded that those moves were for humans to use, not for someone like her, and then tried to see if they would work on a "beast".

The reader/viewer is seeing false memories
During Eren's flashback to being injected with Dr. Jaeger's serum, there's a few timeline issues. Mikasa isn't present, and Eren seems to have spent at least some time with his father after the death of his mother, something which is impossible. However, there is really nowhere else in the timeline it could be placed. It isn't the past, because the mother is dead. It isn't the future, because Eren remembers. So, what if what we've been seeing is what was actually fake? We know there are techniques to alter memory, and we've seen incongruities. Eren is crying in the beginning, but has no idea why it happened anymore. In addition, his memory of Mikasa's hair is different. Is it possible that what we're seeing is just his mind filling in the holes? That he really did leave the city with his father, and without Mikasa?

Eren will fight Mikasa next
Eren's victories over his fellow soldiers have been him gradually becoming stronger. First, he beat Jean, his rival, for fifth place in their year, then he fought Annie, then Bertholt and Reiner. He's been moving up the ranks of their group, and the only person left who's above him is Mikasa. Even more plausible now that we know the Ackerman family has some connection to the conspiracy.
  • No. Eren fights for humanity's survival, not glory. And Mikasa would never try to kill Eren. She loves him too much. Even if the Ackerman family is connected to the conspiracy, Mikasa would likely disown her family name and change her last name to Jaeger, since she is not evil. Stoic and not one to show emotions easily, but not heartless or evil. Besides, she's the deuteragonist. No way they'd have her betray Eren.

Annie dislikes Reiner and Bertolt

(Editing this a little as I felt my previous way of writing it was flawed.)

Contrary to what seems to be popular belief, I never got the impression that Annie's secretly best buddies with Reiner and Bertolt. Rather, her disdain for her mission caused her to dislike them as well as herself. That, or she is indifferent towards them at most. Of course, Annie is hard to read and we don't have enough information to properly analyze the trio's relationship, (they might very well have been childhood friends) but anything after their mission went down seems to have rubbed badly off on Annie, given how she keeps her distance from them 95% of the time and looks annoyed whenever they try and talk to her, even though she's shown being open and interacting with people like Eren and Armin, meaning it's not just a "socializing" problem. Though, none of this is to say their relationship can't improve in the future.
Mikasa's enhanced combat abilities are the result of Eren's Coordinate ability
It's been implied that the Coordinate ability that Eren possesses would also allow him to command normal humans. Indeed, during the incident where Mikasa's kidnappers were killed, she seems to have been affected by his order to "fight". However, one thing you'll also notice during that incident is that it's Mikasa's first display of her superior strength, shattering the knife in her hand and the floor beneath her feet. She herself says that she got full control over her body at that moment. What if she didn't just mean that she stopped shaking? What if it was Eren's order that allowed her to push her body to ridiculous lengths?
  • Eren's Coordinate ability gave Mikasa a version of the Bene Gesserit powers (superhuman control over their metabolism).
  • To throw another idea into the mix, what if Eren using his coordinate ability on her is also the reason why she is so devoted to protecting him throughout the story? After all, everyone notices and remarks on the fact that she seems to care more about protecting Eren than protecting all of the rest of humanity. Even Eren calls her on it at one point. If Eren did use his ability to make her fight in order to protect him, it is possible that her mind has been stuck carrying out that command ever since.

At least one of the human traffickers was under orders from the Government Conspiracy
The real reason they were there was to kill Mr. Ackerman for defiance, kill his wife to clean up loose ends, and bring the kid back into the program to study her.

Levi's cleaning OCD was developed during his time living with Kaney
Levi has already stated that much of his rational was inspired by Kaney, so who's to say that his obsession with cleanliness didn't stem from him as well? Kaney could have had Levi actually get rid of the evidence of his kills and he developed the OCD as a way of coping with what he was doing. Levi had said he lived with Kaney when he was younger, but how young was he at the time?
  • Levi stated outright that he lived with Kaney when he was a child; it's common for children to either pick up habits from their caretakers, or for them to develop habits on their own.
  • And of course, the fact that Kenny is an infamous Serial Killer gives Levi's obsession with cleanliness some much darker connotations...

Marco was a descendant of, or was himself one of "Ymir's People", and that's why he was killed.
The dark hair and freckles are the obvious connection, it's the only thing that Ymir, Ilse, and Marco have in common. We don't know what distinguishes Ymir's People from anyone else yet, but we do know that they have a significant connection to the Titans. It's possible that Marco was murdered not just because he was the most convenient person to kill to get a 3DMG, but because he represented a potential unknown variable in the situation and removing him was just another way to safeguard the shifters' plan.

The Mysterious Woman from Eren's memories invented (or will invent) time travel, and somehow she was erased via time paradox.

Going with the Reset Button theory, the mysterious dark-haired woman that looks like Historia no longer exists. She has a central role in Eren's time traveling ability but neither Eren nor Historia know who she is, despite the resemblance between the three of them. Eren, being a time traveler himself, is capable of recalling her from an alternate future.

The 2000 years from chapter 1 are the same 2000 years mentioned in chapter 55 page 8 (in that chapter, this is claimed to be the lifetime of the monarchy)

The source of the titans is a flying fortress in the clouds
And they'll have to climb a giant bean stalk to get there.
  • What if that's what Isayama is actually planning to put out.

Mikasa's heritage is going to be a big deal later
  • She might not literally be the last of her kind. Believing that Mikasa is the last of her kind means you accept what a bunch of human trafficking thugs said as truth. Fridge Horror when you realize Mikasa may not be the first target they encountered.
  • It's a common cliche to have The Last of His Kind meet another person from his kind, we might see Mikasa going this route.

Being a Titan is a disease.
  • An artificially created one. Further, the thing in the Jaeger basement? It's Grisha's notes and scribbles analyzing the human body, which all point to the same conclusion: Titans are people, just infected. He was trying to work on an antidote, and when he finally finished the first attempt at it, he used it on Eren. You know, that injection that we don't know jackshit about? It was a half-assed vaccine made in desperation. It might not have even worked.

The entire series takes place within closed space.
The Titans are Shinjin, and everyone is a portion of humanity who mysteriously disappeared. And it's all being caused by Haruhi Suzumiya.

The human traffickers were hired by the conspiracy.
Their actual job was to retrieve the escaped Ackerman and bring her (them?) back under government control. The kidnappers would probably have met the same fate either way, but Mikasa might have forged her strong attachment to Kenny instead, which would have been handy indeed.

Levi's realization of the outside world (whatever it might be) was similar to Haruhi Suzimiya's revelation about her being just another human among millions.
Whether it was a revelation of how the world worked or a great secret that could alter the Attack on Titan storyline has yet to be seen. Bonus if said revelation could also be the reason for much of his character being what it is.

There will be some kind of conflict or battle at the 'Factory City...'
The anime mentioned it twice by name (I believe) in the supplementary material. It's mentioned that it's the only place (for some reason) that can produce the hardened steel used to slay Titans. Not having that would put humanity in a rather dire circumstance considering how much of that the military is burning through nowadays, and it would be sort of weird to mention it at all, let alone more than once like that, for absolutely no reason... maybe it's a future Titan-Shifter target?

The Mysterious Woman is a younger Carla Yeager
They both look relatively similar to each other, with the differences being explained by the first time we see the Mysterious Woman being some time before she met Grisha, and the scene with Historia a year or two before the fall of Wall Maria.
  • That makes no sense, though. Eren and Historia are roughly the same age, and it's unlikely a homemaker with a son prone to getting into daily trouble would be sneaking off to the Interior to secretly tutor Historia. Remember that the Walls are roughly the size of a country like France, so travel between Wall Maria and the Interior took at least a day or two. Besides that, if it were Carla I think Eren would recognize his own mother.

Levi asked Kenny to teach him how to kill
Just because of the possibility that Isayama just might throw this curve-ball. Basically, Levi witnesses Kenny killing the Military Police but instead of being scared, he is amazed at how easily Kenny did it. He has his own grudge against the MP because they wronged him in the past (whether beating him up, framing him for a crime and locking him away or killing someone he cares for for kicks or otherwise)- whichever way it falls, Levi wants to kill them, similar to how Eren wants to kill the titans. Kenny initially refuses because he REALLY didn't want to have to look over some mangy street kid as he had enough on his platter as a wanted serial killer but Levi keeps following him like a dog until he eventually relents.

And makes Levi regret ever wanting to kill.

The Walls were always meant to come down.
The Walls were made before humans reached them, and are filled with giant Titans. They are also full of resources needed for humans to defend themselves from lesser Titans. Every attempt to bring down the Walls has been done by the Shifters, who are enemies of normal Titans too. One of the few people in the know, Ymir, mentions how the interior is going to become a hell on earth and wants to take Krista outside the Walls before that happens, specifically noting that avoiding this is impossible. Later, upon seeing Eren has the Coordinate, Ymir says that there is a future within the Walls now. My guess? The Walls were built to corral humanity, give them a safe place to grow in numbers, and after a certain point they will collapse and the Titans within will devour everyone. This future can be averted by Eren, now that he has the Coordinate he could order the released Titans away from the humans and save them all.

Marco will be the unknown Titan Shifter that the King will send to eat Eren.
Forgive me if some one already made this guess but... consider the fact that the circumstances of his death were suspect, Annie "stole" his gear, when in reality he could of simply given it to her in exchange for her help in faking death, add things on like the fact he wanted to join the military police specifically to serve the king, it starts to fall into place.

When Marco said he planned to join the Military Police and serve the king, the king he was referring to was Rod Reiss.
Basically, Marcos' family has been acting as bodyguards to the Reiss family for generations, even before the walls existed.

Either Ymir or Historia will die
They're canon now, and as we all know, happiness cannot be allowed to exist in the Attack on Titan universe.

The Titans in the Walls are Eren's (and possibly Historia's) ancestors, and they went willingly.

The Jaeger family is a branch of the Reiss family, which explains the uncanny resemblance between Eren, Carla, the Mysterious Woman, and Historia.

We know that there are Titans in the walls, but how did they get in there? They were shepherded in by a Coordinate! When the Titans first started running amok and eating all the humans and the remaining human population escaped, the Reiss family realized that the only way to stop it was to build walls tough enough to keep them out. The Jaeger family volunteered to become the Walls, willingly becoming Titans so they could be used as the foundation for a sanctuary for the rest of humanity; that's why the Reiss family is now the ruling dynasty, because it was their family that saved humanity from being completely devoured.

Being the "coordinate" has nothing to do with the ability to influence Titans.

It refers to a time coordinate. Eren's abilities are a coincidence, the important thing is that he is connected to a specific point in time that can be used in time travel somehow.

Lord Reiss is NOT the true king.
I don't have much evidence for this, except that when Historia is explaining her past, her father tells her that she's going to come live with him and tries to take her to safety. He is then seemingly stopped by the group of men. If he is the real king and they are his servants, how did they have the authority to stop him from doing whatever he pleases? Perhaps Lord Reiss is a fake king as well, and there is a third king(or queen, perhaps even a group of people) who is really the one in charge, and Kenny the Ripper serves them.

Kenny actually treated Levi decently when they lived together.

Sans teaching him how to kick ass- that would definitely involve some manner of injuries.

Hey, it's possible.
Ackerman is to the Walls as Freedman was to 1860's America.
In 1860's American freed slaves often had to choose a last name and often chose "Freedman" or "Freeman" to make it clear that they were free. This became a default name of sorts. What if Ackerman became a semi-default last name for orphans or those who did not have a known last name?

The Memories Rod Reiss had Eren remember in Chapter 62 were fake
He wants the Co-ordinate ability and created those fake memories as a means of manipulating Eren into going along with his plan. Historia is unaware of this and believes they are just triggering a recollection of what he forgot. His daughter did have the ability to erase memories. The reason Rod doesn't use this Fake Memory Power frequently is because he can't erase memories meaning people will remember him and figure things out. He has used this ability at times though. Remember, in Historia's story about her past, he told her that she's going to come live with him, tried to take her to safety but was stopped by a group of men even though he is the true king. As the true king, those men could not have stopped him. It never happened, he just met up with her and made her think that it did.

The 2000-years monarchy is a lie
Planted by the mind-wipe during the creation of the city to manipulate the people into obedience.

There are human settlements with an approximately 20th century-level of technological development
Sooner or later a titan will cough up a car or something similar and everyone from the Walled City will be totally baffled about what the devil they're seeing.

The inhabitants of the city read and write a Cypher Language
The excerpts of their script we see don't look like anything that matches the European style of everything else. The government outlawed the use and teaching of Roman letters and other existing writing systems in order to make sure that if anyone did find an old document (or... herring), they wouldn't be able to read it. They probably still speak whatever they did before, since replacing that would be significantly less practical and it likely doesn't matter so much. :Or they were brainwashed into using this language at the beginning of the Walled City.

The origins of the Wall Titans.
The Wall Titans, as we all know, use their crystallization powers to create the massive walls. The question is: why? Simple. They're all rebels. Every evil faction has a schism at one point or another. These Titans all had a conscious and created the walls to stop their former-comrades-turned-enemies from destroying humanity. One day, they will come out of hiding and there will be a massive Titan Civil War in which Annie, Reiner, and Bertolt pull Heel-Face Turns, Jean and/or Connie die, and Eren kicks ass.

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