The ubiquitous "gotta shoot 'em in the head" scenario. For many different reasons, The Undead throughout fiction are vulnerable only to Boom, Headshot. Kinda like how (many) vampires are only vulnerable to getting stabbed in the heart. Fortunately, removal of the head also works on people who are not a member of the walking dead, so you don't have to worry about it going out of fashion as a killing method. This may have a somewhat scientific basis, as a zombie would have a lowered to nonexistent pain response, and would most likely not stop attacking until its nervous system has been effectively severed. Of course, if you wanna get scientific about it, excessive blood loss or organ damage should stop a zombie just as well, but accepting this rule is often a part of suspension of disbelief.
Is a consequence of a Cranial Processing Unit.
An extension/special case of the Chunky Salsa Rule. Sister trope to Boom, Headshot and Attack Its Weak Point. It is distinct from them, however; Boom, Headshot is when a headshot is the most efficient way to kill something, whereas this trope is where Boom, Headshot is the only way to kill something; Period. Also closely related to and frequently overlaps with Decapitation Required, but the latter trope usually applies to zombies, vampires etc with a supernatural origin as opposed The Virus.
Most songs about zombies tend to lampshade this trope.
Claymore. Due to Yoma's amazing Healing Factor, a quick kill is absolutely necessary, and even then, it's preferred to completely tear their corpses to bits. Ophelia lampshades this, telling the Awakened who breaks her neck that you need to behead Claymores to be sure.
In Mermaid Saga, some people who eat mermaid flesh turn into zombie-like monsters and some become immortal. In either case though, the only way to permanently kill them is to decapitate them.
This is how you kill vampires in JoJo's Bizarre Adventure - the mystical artifact that creates them does so by altering their brains, making destruction of the head the only way not involving sunlight or Hamon to get rid of them. (Decapitation just creates a pissed-off vampiric head, so you have to destroy the head entirely.)
Subvered in Fullmetal Alchemist with the zombie-like "mannequin" homunculi. While destroying their heads does keep them from biting, they stay alive and it's much easier to disable them by injuring their legs.
Naruto gave us Hidan who, despite being human, uses a forbidden jutsu that makes him immortal and the only way to even slow him down is to decapitate him. This won't necessarily defeat him permanently, as his head can still be reattached by Kakuzu (and possibly other methods). Since Shikamaru left his body in pieces buried in a ditch in the middle of nowhere, his severed head is shouting obscenities until he eventually starves to death.
Marvel Zombies can only be killed by destroying their brains. Decapitation just leaves an irritated head and a lifeless body (as shown by Zombie Wasp and Headpool).
Except for Earth-Z's Electro, which was his headless body walking around.
In the Naruto fanfic Time and Again Naruto fights a rogue ninja with incredible regenerative powers. He's able to take him out of action by cutting off his head, but is Genre Savvy enough to watch it for a few minutes afterwards and sees the flesh of the neck wriggling as it begins to try and grow a new body, so he pulverises the brain, which finally kills his opponent once and for all.
"In the brain and not the chest. Headshots are the very best!"
Played with in Scream 3. The killer is wearing a bulletproof vest under his Ghost Face costume, so Dewey has to shoot him in the head to kill him. This is after Sydney stabbed him in the chest with a stiletto, giving you a rather interesting take on the "stake-through-the-heart-and-cut-off-the-head" method of killing a vampire. Lampshaded earlier in the movie when Randy, in his postmortem trilogy rules video explains that the killer is going to be supernatural this time around, requiring some special means to deal with him.
Averted in Outpost. The mercenaries are watching a film from World War II and realise the Nazi commander shown is their Sole Survivor. One of the mercs immediately walks into his cell and puts a bullet in his brain. "Is he dead?" / "His brains are all over the wall. That's good enough for me." Cue Mass "Oh, Crap!" when the 'dead' Nazi lifts his head to look at them and the lights go out.
Zombies from The Zombie Survival Guide and World War Z are vulnerable only to headshots because the virus causes radical mutation, making everything but the brain completely vestigial.
Partly averted. Removing the head renders the zombie harmless, but not destroyed. The head is still 'alive' and can still bite. In fact, the Record Attacks comics recount a rite of passage involving spending the night locked in a room full of moaning zombie heads (the author does note that it's impossible for them to moan if they lack a torso, although it has been suggested the moaning is psychosomatic; it's expected, so it's fervently imagined to happen).
A soldier also recounts in World War Z how it is entirely possible to shoot the skull but miss the brain and, thus, need more than one headshot to finish the zombie. The author includes this as a warning for people not to panic that the zombie is invincible if one shot doesn't do it... unlike the men in the soldier's squad that day...
The Vampire Counts book, however, suggests that when you "wound" a zombie, you're actually smashing it into enough pieces that it can't do anything - which, given the state of decay involved in one of these things (they're literally held together with fence posts and nails), is easier than you might think.
Decapitation is actually the only way to kill a vampire in Bram Stoker's original Dracula. (Buffy-style staking is kind of a Plot Tumor of the Dracula-derived vampire mythos; driving a stake through the heart is used to immobilize the vamps in Dracula so it's easier to take the head off.)
Lampshaded in Brains: A Zombie Memoir.
Averted in Counselors and Kings- it's explicitly stated that removing the head does not destroy a zombie, though it does blind and deafen it (since the now-headless undead has no eyes or ears). Magic or completely destroying the corpse (through dismemberment or fire) is what kills it).
In Undead on Arrival, it's a specific part of the brain. They don't think, so they don't need the front, they don't remember so they don't need the top, but they can bite. So to kill that zombie variant, you have to destroy the reptile part: the medulla.
The Sandman Slim novel Kill the Dead has a variant. Permanently killing a zombie requires the removal of the entire central nervous system.
Those infected with the Shaod in Elantris can only be killed by removing or destroying the head.
Geeks in The Walking Dead; downplayed when the group cuts the head off a walker, but the head is still alive. Daryl comes along and shoots the head and comments that only a headshot through the brain will put them down. This is also true in the Graphic Novel series it's based on.
Later on in the series, Philip Blake finds himself having to explain this to someone who doesn't understand why shooting them in the chest hasn't been working for her.
The Community episode "Epidemiology" has an outbreak of zombies. Chang shouts "You have to destroy the brain!" after Rich and Britta turn into zombies while in their safe area. Unfortunately, his attempts to follow his own advice directly lead to a broken window (breeching their own defenses) and allowing the zombies to grab and infect Annie.
In the Agents of S.H.I.E.L.D. episode "Beginning of the End", Garrett helpfully points out that he isn't dead yet since Mike Peterson didn't cut off his head. While Garrett is gloating about how invincible he is, Coulson uses a disintegrator ray to destroy Garrett's brain, along with the rest of him.
Averted in Torchwood: Miracle Day. The premise of the mini-series is that no-one is dying, anywhere in the world. A suicide bomber is still alive despite being, well, exploded. Jack Harkness suggests that they cut off the head to see if it'll still live. They do so, and it looks like the bomber is finally dead... And His Eyes Open.
The Resident Evil games zig-zags the trope, but until a certain point, it's played straight with T-Virus zombies. Shooting them or their crimson heads "evolutions" anywhere else on their bodies does take them out of the fight for a good while, but only until they mutate to the next form. The crimson head's next form, the Licker, averts it quite spectacularly, in the sense that its (fully exposed) brain isn't even a weak spot - its (also exposed) heart is.
In Resident Evil 2, Chief Irons calls this trope out nearly word-for-word in regards to the mayor's daughter, an apparent zombie victim killed by Irons himself; he just blamed the zombies.
This actually gets subverted in Resident Evil 4 and 5. The "zombies" of the game, the Ganados/Majini can often be stunned by shooting them in the head, but doing so brings risks. Namely, destroying their heads may release difficult to kill Las Plaga, some of which can kill the hero in one hit. The best option is to shoot them in the body, legs or head once to stun them, then use melee attacks.
The Dead Hands boss in Ocarina needs to be struck in the head, although it's unclear if DH is undead or something else.
The zombies in Cold Fear. The Exocell parasite nests in the cranium, feeding on the brain.
Subverted in Dead Space. A headshot alone will not kill a Necromorph, unless you've cut off at least one other limb. One of the gameplay features is called "strategic dismemberment", where removing or destroying certain parts has different consequences, depending on the Necromorph. The best example of this would be the Pregnant; shooting its arms is the best bet, but a bodyshot will release the Swarmers in its belly and they will probably kill you.
The zombies from the Half-Life franchise are best dealt with this way. Although damage to the body will severely wound them, they never truly are down until their Headcrab parasites are bleeding yellow.
Minor zombies in the House of the Dead series can be taken out like this. Bosses have their own weak points (although some are in the head).
Touted by some humans in Fallout 3 on how to kill ghouls ("zombie" being used as a slur for ghouls), even though they die from normal damage just like any other creature would. One particular ghoul wants you to kill certain anti-ghoul humans, and will pay you more for killing them with a head shot. However, only one of them is anti-ghoul, the others have a key for a bunker that has the T-51b, and you can keep the keys and take the T-51b for yourself. Or, if you're playing a Good game, you can talk them out of their keys and hand them to Mr. Crowley for the same money without ever firing a shot.
Invoked in Eternal Darkness with Ulyoth Zombies. One has to decapitate them or they will go Action Bomb, however, zombies of all four Ancients will still fight without their heads (and will be momentarily stunned, comically patting their neck stump as if to say "Oi, who turned off the lights?").
Used in the final boss battle in the Marine storyline for the 2010 Aliens vs. Predator videogame.
Not a zombie example, but in Star Trek: Bridge Commander, destroying another ship's bridge (which can be considered a brain), will cause the entire ship to explode, even if the rest of the ship was fully intact. This is subverted with Klingon ships, as it's very easy to blow off the front of it and would be game-breakingly easy to defeat them. Though you have to wonder who's flying the thing...
In TimeSplitters 2 the quickest way to kill a zombie is to shoot off its head
Zig Zagged in Halo with regard to the Flood. The after becoming flood, the head is useless and they can lose it with no real problem. But the vital zone is their chest, where the infection form is. After having their chest destroyed, the combat form is effectively killed... until another infection form comes along and reanimates it. The only way to ensure no reanimation is to destroy the corpse (most efficient way is to burn it, but continually shooting/meleeing it also works).
Invoked in Urban Dead; zombies killed by headshot take a 5 AP penalty to their "Stand Up" ability.
Invoked in the Borderlands DLC "The Zombie Island of Doctor Ned". True to their humanoid shape, the zombies' weak point is the noggin. Granted, this is also true of every human enemy.
In the Fallout: New Vegas DLC Dead Money, the Ghost People do not die unless one limb is destroyed; it doesn't have to be the head, though it's undeniably convenient. If not killed in this manner, they just revive with full health within a minute. Fortunately, you can still amputate during this period. If you have the super-mutant Dog with you, he can eat them instead, which is also a permanent death, and can teach you to kill them without needing to take a limb.
In Sniper Elite: Nazi Zombie Army, if a zombie isn't killed by a headshot, it has a chance of rising again after a few seconds. Headshots are also instant kills on the most common enemies, so it's in your best interest to aim for the head as much as possible so as to conserve ammo. There are also minibosses and bosses who can only be hurt by headshots; naturally, they take a whole lot of headshots to kill.
In Contagion, taking a headshot from a bullet or charged melee weapon will drop anyone, zombie or not.
Downplayed in The Last of Us; the Infected are most vulnerable in their heads, since the fungus that takes over and sprouts from their brains renders their skulls somewhat fragile, but enough shots to centre mass will do the trick as well. Most of Joel's mêlée takedowns on Infected have him either bashing their heads open against a hard surface (or with his weapon if he's holding one) or stomping on them. It seems as though gunshots to any part of the body can kill an Infected, but anyone hoping to take one out with a mêlée weapon or bare hands needs to aim for the head.
Zigzagged in Plague Inc. Necroa cannot reanimate headless corpses until you evolve Cranial Metastasis; which causes brain tissue to develop in the chest cavity. Another symptom causes the skull to thicken, making it harder for survivors and Z Com/ Black Water soldiers to kill them by headshot.
Both lampshaded and averted in The Adventures of Wiglaf and Mordred. One character (Gawain) is a Revenant, an intelligent zombie. The first thing that happens to him is a headshot. Arthur, who witnessed the event then calls foul claiming Gawain can not possibly be a zombie — only to be corrected:
Arthur: You're not a zombie. Everyone knows you take them out with a shot to the head. And you're still standing. Gawain: Have you ever killed a zombie? Arthur: No. Gawain: Met one? Arthur: No. Gawain: Then, how exactly do you know that actually works?
The zombie wranglers of Zombie Ranch declare the front part of the brain "don't matter", but that destroying the rest is a sure way to put a zombie down or prevent an infected person from turning. If you stay away from the brain you can kill an infected person and they'll still rise again later, which leads to this fateful decision.
Subverted in The Order of the Stick, when Roy decapitates an undead dragon. Xykon points out that he controls it with his mind and it doesn't need eyes, ears or a brain to follow his commands. In fact, the main thing Xykon is annoyed by is that the dragon no longer has teeth to bite with.
Invoked in Tasakeru. During a fight with a Made of IronRevenant, Commander Nadeshiko wonders how invincible he'll be with a split skull. She doesn't get a chance to find out.
Discussed almost like a mantra in After Hours when the gang discuss zombies. Subverted by Katy, who's apparently unfamiliar with the rule and mumbles something ending with "... extract Brian."
The Flash: How do we fight it, or them? Mophir: Two ways. Pure light from Mophir's gem drives evil spirits back into Dark Heart. The Flash: Great. What's the second way? Mophir: Separate host head from body.
Frylock: I'm sorry you had to see that, Meatwad. But in order to kill a zombie, you have to separate the brain from the spine. Headless Shake: Hey I just heard, like, a pop. Did you guys hear that? Frylock: (throws away axe) Goddamn Wikipedia!
With one exceptionnote Mike the Headless Chicken, either of these methods will kill any animal you could possibly encounter in real life note well, okay, aside from the ones that don't have heads.. (Effect may be delayed in some insects and crustaceans.)
Interestingly, in some insects, such as the cockroach, the only reason decapitation eventually kills them is that they eventually starve to death. No head means no mouth.
This is because a large number of insects have a secondary nerve cluster in the thorax just behind the head, which essentially acts as their backup brain. In an extreme example, this is useful. The common Praying Mantis male often gets its head eaten first by the female during mating but is able to continue copulation - removing the head simply removes its limitations and allows the male mantis's body to continue mating until it dies or is completely devoured by the female.