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[[caption-width-right:250:[-Okay! So you shoot down the UFO and then land and take out the surviving aliens. I think I got thi-- [[DifficultySpike HOLY]] [[NintendoHard CRAP]]!!!!-] ]]

The brainchild of Julian Gollop and other assorted Creator/{{MicroProse}} personnel, ''UFO: Enemy Unknown'' is a strategy game produced in 1993 and unleashed upon the European gaming public. A year later, it jumped the pond to grace American players as ''X-COM: UFO Defense''[[note]]name changed due to a naming rights conflict with an obscure 1989 flight simulator by subLOGIC called ''UFO''[[/note]].

''X-COM'' puts the player in command of an e'''X'''traterrestrial '''COM'''bat unit charged with protecting Earth from an alien threat, while also managing resources and researching captured alien technology in the process. The game consists of a hybrid of RealTimeStrategy (improving X-COM's overall condition and catching [[FlyingSaucer UFOs]] by either storming their landing sites or shooting them down) and [[TacticalTurnBased Turn Based Tactics]] (exploring crash sites, stopping terror attacks, and defending and assaulting bases).

The ''X-COM'' legacy is a not solo act, however. While Gollop's team set to work on a sequel called ''X-COM: Apocalypse'', an in-house crew at [=MicroProse=] beat him to the punch with ''X-COM: Terror from the Deep'' in 1995, a MissionPackSequel created to satiate player demand for more alien-assaulting action. ''Apocalypse'' was released in 1997 to average reception, and included the option to play [[RealTimeStrategy in real-time.]] The last days of [=MicroProse=] (and its acquisition by Hasbro Interactive) saw ''X-COM'' have two {{Genre Shift}}ed offerings: ''X-COM: Interceptor'' (1998), which kept the base management elements while swapping out the strategy missions for space-bound [[SimulationGame Flight Simulator]] action, and ''X-COM: Enforcer'' (2001), which ditched the strategy part outright to make a FirstPersonShooter running parallel to the timeline of ''UFO Defense''.

After that, the possibility of a future ''X-COM'' game became uncertain due to the licensing passing between various companies. The earlier games spawned a number of mods and remake attempts. Various [[SpiritualSuccessor Spiritual Successors]] also emerged, such as ''[[VideoGame/UFOAfterblank UFO: Aftermath]]'' and its sequels ''Aftershock'' and ''Afterlight'' (unrelated to the original ''UFO'' name, listed above). Another, ''Rebelstar: Tactical Command'', came out on the GameBoyAdvance along with ''Laser Squad: Nemesis'' (in and of itself a sequel to ''X-COM'''s own predecessor ''Laser Squad'') and ''[[http://ufo.ufo-extraterrestrials.com/ UFO: Extraterrestrials]]'' which is almost an exact remake of the original game. Fans have also made their own remakes, most notably ''[[http://openxcom.org/ OpenXcom]]'', ''UFOAlienInvasion'' and ''VideoGame/{{Xenonauts}}''. [=OpenXcom=] itself has enabled total conversions like the VideoGame/{{Piratez}} mod.

In 2010, [=2K Marin=] announced that they were developing a ContinuityReboot of the series: a FirstPersonShooter set exclusively in the USA in TheFifties. The fandom's negative response to the reboot was heavy enough that, instead, Firaxis Games (a subsidiary of 2K and developers of the ''VideoGame/{{Civilization}}'' series) was tasked with creating a new game closer in-line with the original ''X-COM''. The result was ''VideoGame/XCOMEnemyUnknown''. The original FPS reboot was {{Re Tool}}ed into a third-person tactical shooter called ''VideoGame/TheBureauXCOMDeclassified'', intended as a prequel to ''Enemy Unknown'' (the game's three separate iterations [[http://www.ign.com/videos/2013/05/10/the-evolution-of-the-bureau-xcom-declassified are documented here]]).

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!!''X-COM'' provides examples of the following tropes:

[[foldercontrol]]

[[folder:General]]
* AdventureFriendlyWorld: See CrapsackWorld. This works out fine as the backstory of a hyper-lethal squad combat game: the utter monstrosity of your enemy means that as long as any humans survive, the NonEntityGeneral can always find vengeance-crazed replacements for troops lost in combat, or at least someone willing to die for a carrot, and there is an unending supply of alien baddies to kill, capture and vivisect. But taken out of context, X-COM is essentially sending unaccountable death squads against an enemy that can never really be beaten without desperate measures.
* AirborneMook: The Floaters, their equivalents in the sequels and various terror monsters.
* AlienAbduction: Floaters and Sectiods practice this. They even have a specialised UFO called the "Abductor".
* AlienAutopsy: The player's scientists can perform these.
* AliensAreBastards
** The first two games played straight.
** Would have been subverted with the canceled ''Alliance'', where the stranded X-COM team form an alliance with the friendly aliens against the hostile ones.
** Downplayed with in ''Apocalypse''. The new aliens are certainly bastards, but some of the ones we've been previously familiar with now co-exist peacefully with humans.
** And then subverted again in the reboot: [[spoiler:Turns out the Ethereals are [[WellIntentionedExtremist Well-Intentioned Extremists]]]].
* AlienBlood
** Green and Yellow seems to be most common ones.
** Ethereals have silver blood (though it's dark red in its Autopsy picture).
** Aquatoid blood is orange.
** Lobsterman blood is teal.
* AlienInvasion: Duh.
* AliensAndMonsters: Mostly aliens, but their Terror Units are often engineered (genetically or otherwise) to either capitalize on their owner's strengths (Chryssalids having weaponized the Snakemen's rapid asexual reproduction, for example) or cover their weaknesses (Sectopods distracting the enemy with conventional attacks while their Ethereal masters make with the MindRape). Except for the [[GoldfishPoopGang Silacoids and Celatids]], which (given that their counterparts, the Mutons, don't really ''have'' weaknesses) don't really do anything.
* AliensStealCattle: Several missions involve "Harvester" [=UFOs=] sent to meet the aliens' carnivorous needs. [[IncrediblyLamePun They are, of course, equipped with ]] CowTools.
* AllYourBaseAreBelongToUs: A major gameplay element; you have to design your hidden underground bases with defense in mind, since aliens ''will'' eventually find and attack them. Later in the game, of course, you get to do the same to them. (Or earlier, since unless you're doing a really bad job, you'll find some alien bases before they find yours.) Innovations in technology will eventually provides missile and psychic defenses to fend off [=UFOs=] or, in the case of the latter, cloud detection of the base completely.
* AllianceMeter: ''UFO'' and ''TFTD'' has this in form of Funding Nations. Scaled up in ''Apocalypse'' with 25 factions waging corporate wars for political and economical power in Mega-Primus during the alien invasion.
* AlmightyJanitor: Your newly hired and unranked recruits, thanks to their randomly created stats, are potentially capable of being incredible marksmen, MadeOfIron or -- when you have researched a Psionic Laboratory -- mindraping any alien they see into commiting treasonous and suicidal acts of violence against their own side. (But more likely they're completely useless and you'll have to sack 8 out of 10 when you finally get their psi evaluations.) If you know what the limits are for a fresh recruits stats (for example, they can start with 40 to 70 time units), then you'll quickly realize that most of your recruits literally ''are'' cannon fodder, being at the bottom rung of effectiveness.
* ApocalypseHow: The result of failing to defeat the aliens (and sometimes even when succeeding). See the more detailed AP examples in each games respective sections.
* AppropriatedTitle: The series started as ''UFO: Enemy Unknown''. It had to relabel itself ''X-COM'' when somebody complained there's already a game called ''UFO''.
* ArmchairMilitary: The "rear commander" tactic. Since high-ranking officers contribute to the Morale level (and significantly hamper morale when they get themselves killed), it will behoove you leave them behind in the Skyranger. The officer will continue to gain EXP and receive promotions, with the rookies and squaddies stuck doing all of the legwork.
* ArmorIsUseless:
** A soldier with maximum health and the best armor can still be killed in one shot if the damage roll is high enough. Even the best armour has at best a 50/50 chance of stopping a Heavy Plasma shot, though it does make troopers immune to many human weapons and it provides much better protection than most alien units have. To clarify, X-COM soldiers take 0 to 200% of the listed damage from firearms; 50 to 150% from explosives. Unarmoured troopers can survive several heavy plasma blasts and take absolutely no damage... only to be offed by a single pistol shot the next turn.
** The manual does state that the default coveralls ''do'' offer excellent protection from modern weapons, although it skimps over the plasma resistance capability...
** ZigZagged in ''Terror'': The front of the Ion Armor can let soldiers take point blank Sonic Cannon blasts or Lobstermen's claws and take no damage as long as it hits the front armor (unless random chance screws you over), but still played straight with Bio Drone explosions and Tentaculats. Also, Ion Armor is much weaker at the sides, meaning the alien waiting around the corner can easily take out your soldier if you're not careful.
** Averted in ''Apocalypse''. Megapol Armour is fairly competent, particularly against light friendly fire and early disruptor weapons, but is terrible against devastators. Marsec's flying armour is weaker but allows flight. On the other hand, X-COM manufactured "Disruptor Armour" transforms soldiers into [[TheJuggernaut nigh-unstoppable]] death machines who can practically ''waltz'' through multiple explosions without even taking a mortal injury. The shields certainly help, though.
* ArtificialStupidity: The civilians in Terror Sites. They will run through a door, back through it, then back AGAIN. That is if they are not running into the middle of fire fights, because the natural place to stand in a military operation is ''directly in front'' of the man with the laser rifle.
* ATeamFiring: Most recruits will hit [[http://static.tvtropes.org/pmwiki/pub/images/AW-XCOMAccuracy.jpg everything except the aliens]].
** This is especially evident when using Auto Fire (which makes soldiers shoot 3 less-accurate shots in rapid succesion). Agents can even be firing an ''accurate'' weapon like any of the Rifles at point-blank on full auto and have the shots knock down the walls and trees behind an alien without even grazing it. On the other hand, Aimed Shot (a single more-accurate shot that uses up more of your Time Units for the turn) is actually ''very'' effective with enough training in Firing Accuracy.
** Auto shot is preferable early on when you know even the soldier's aimed shot will most likely miss. First, it has a chance to hit aliens multiple times, stray bullets will sometimes hit other aliens (even those you didn't notice), and if the alien can see you, a single burst will only trigger reaction fire once as opposed to aimed or snap shot which trigger reaction on every shot. Once you have laser weapons, you most likely use auto shot at every opportunity.
** [[http://www.ufopaedia.org/index.php?title=Firing_Accuracy_Testing Experimentation]] has shown a few oddities with accuracy in the first two games. In particular, the quoted % accuracy is actually understated a majority of the time. The reason for this is that a "miss" is not actually a miss, but rather a random deviation applied to the bullet. If you're lucky (or at point blank range), this deviation will be small enough that the bullet hits anyway.
* AttackDrone
** Cyberdiscs and Bio-Drones.
** ''Enforcer'''s protagonist. Recursively, he too can get an AttackDrone.
* AuthorityEqualsAsskicking
** Aliens only in the first two games.
** Not necessarily true with human soldiers, depending on whether the player has the officers on the battlefield getting exercise along with the other soldiers or leaves their muscles to atrophy in the back of the Skyranger.
* AwesomeButImpractical: Several weapons and base components, either due to how easily their replacements can be researched or by being {{Nerf}}ed by the game mechanics.
** The Sonic Cannon of ''TFTD'' is the biggest offender. It can firing snap shots or aimed shots and inflicts about 15% more damage than the Sonic Blasta-Rifle. However, firing a snap shot will take ''half'' of your time units, and an aimed shot requires ''75%'', meaning you have almost no ability to manuever. In addition, it has five fewer rounds per clip, and is so heavy you're limited in what else you can carry. The Blasta-Rifle is superior in every way ''except'' damage and, to much a lesser extent, accuracy, but since you can fire two snap shots ''and'' move with the Rifle, the slight damage increase the Cannon offers is negligible. Fortunately, [[ArtificialStupidity the computer will use the Cannon, and all it's attendant problems, exclusively about two-thirds of the way into the game.]]
** The Griffon Tank in Apocalypse: Huge, has a BFG. Awesome stats for something you can get at the start... but because of a coding decision, will be destroyed if the road under it gets damaged, no matter what its current health is. Also, it's SLOW; flying vehicles can easily avoid it.
** Heavy infantry-carried weapons, at least in the first two games -- launchers such as the Auto-Cannon can inflict serious amounts of damage, but their weight means rookies will have problems carrying them. In general, equipping rocket launchers, auto-cannons and the like to inexperienced soldiers (which are all you've got in the early-game!) will mean they may not have enough [=TUs=] to actually fire them effectively or, not to mention, ''move''.
*** A notable exception is the Blaster Launcher, which fits under [[GameBreaker/XCom a completely different heading]].
* AwesomePersonnelCarrier[=/=]CoolPlane
** The Skyranger VTOL jet.
** ''Terror from the Deep'' introduces the Triton, a submarine equivalent of the Skyranger.
** The ultimate troop transports (the Avenger, Leviathan, and Annihilator) are also the ultimate fighter craft!
* {{Badass}}: Any human who lives long enough. Everybody starts out as a RedShirt, but over time they can become absolutely terrifying, some even capable of single-handedly slaughtering entire alien craft full of enemies in a single mission.
* BeePeople: The Sectoids and Aquatoids are described as such. The ''Apocalypse'' aliens as well.
* {{BFG}}: The series is full of them, from the Heavy Cannon to the Rocket Launcher to the Heavy Plasma to the Blaster Launchers and their counterparts.
* BizarreAlienBiology: Autopsy results. Some are fairly mundane, some are heavily cybernized, and some who by all means should have been dead when they were alive.
* BlackBox: Even when research is done, there are still something that bugs the scientists, usually the autopsies of the more exotic aliens and miscellaneous tech. They sensibly ignore it rather than taking the (extra) time to figure it out.
* BlackMarket: Though not explicitly stated in the games themselves, the manuals say that X-COM sold goods to buyers of questionable reputation, and when the times became rough they even stopped bothering asking about who's buying as long as they had the cash.
* BodyHorror: Chryssalids, Bio-Drones, Tentaculats, several of the ''Apocalypse'' aliens, and so forth.
* BoringButPractical: Laser pistols and rifles, which are cheap to manufacture, fairly accurate and use up no ammunition. Thanks to decent rate of fire and damage, they remain useful for entire length of game, with sole exception being Mutons.
* BottomlessMagazines
** Only for laser weapons in the first game, the major reason they are so good. Hideously averted for everything else (see EasyLogistics below) except for aircraft and [=HWP=] energy weapons and even those just have very large magazines (100 or 255).
** While the first game went with the "more powerful weapon = more ammo in clip" method (the plasma pistol has 15 rounds, while the heavy plasma has 35), ''TFTD'' decided that more powerful weapons need smaller clips (the Sonic Pistol has 20 rounds, the Sonic Cannon has ''10'').
* {{Brainwashed}}: The common state of victims of Ethereals, high-ranking Sectoids, and their successors. Often, they're [[BrainwashedAndCrazy also crazy]].
* BulletholeDoor: Great for reducing the effects of drone blockage during Terror Missions. Busting through the walls of [=UFOs=], however, will take well-placed/lucky plasma holes (interior) or Blaster Launcher shots (exterior). Or [[BigBulkyBomb lots of plastic explosives]], but that's [[ThereWasADoor another trope entirely]].
* CharacterLevel: The Soldier ranks are a lesser example of this, and which soldiers get promoted is out of the player's control (though ''[=OpenXcom=]'' has a mod for manual promoting) based on the performance of the soldiers and how many of them are in your employ. The higher the rank, the better the morale the soldiers have. Getting the high ranking officer killed will also make the morale drop like a rock.
* CharlesAtlasSuperpower
** With sufficient combat experience, a soldier can eventually beat out a tank in health, movement, accuracy, etc. Oh, and tanks can't get those nifty Psi abilities.
** [=XCOMUtil's=] modified [=HWPs=], on the other hand, are absolutely terrifying, and are capable of reliably hitting an enemy from a considerable distance away. And if they miss, well, that's why you use the Rocket Tanks... until you get the Fusion Ball Tanks. Which can never miss, unless you're bad at setting in the missile course.
** Also demonstrated by Commander units on the enemy forces, particularly in ''X-COM'' and ''TFTD''. Your average Floater, for example, dies if you so much as glare at it. Floater Commanders can take several rifle rounds to bring down, on the other hand. Rank distinctions were removed in ''Apocalypse'', however, though enemy stats could vary greatly.
* ChestBurster: Chryssalids and Tentaculats.
* CloneArmy: Alien autopsies reveal that, with few exceptions, most of the aliens are clones.
* LesCollaborateurs: Repeated screwups in a particular funding nation or outright political manipulation thereof by the aliens can result in said nation cutting its remaining funding to X-COM and signing a nonaggression pact with the grey bastards. Most annoyingly, when you spot a UFO or USO on a "diplomatic mission", if they've landed, you're already too late. You can assault the aliens, kill every single one, loot their ship and prevent any further incursions into that particular nation's airspace, and at the end of the month be told that they've signed a non-aggression pact with the aliens. Even if the ship was only on the ground for five minutes.
* TheComputerIsACheatingBastard: Most prevalent in ''UFO Defense'', where aliens don't suffer from Fatal Wounds unless they were inflicted under previous mind control, magically know the entire map (and your soldiers' positions) after Turn 20, and can target any of your soldiers as soon as just one is in visual range (particularly rage-inducing with Ethereals' psi-spamming). Even so, it's possible to fool them by bringing a psi-decoy with low mental defences and no weapons to suck up all their psychic powers, as they are always going to target the people wth the weakest minds.
* ColonelBadass: The Commanders of both sides, though the actual rank of Colonel is the second highest. ''TFTD'' has the Captain and Commander as the highest and second highest ranks respectfully.
* CombatMedic: Anyone with the medkit, and boy, you're gonna need them. You can also pick up a downed soldier's own medkit and use it on him, as knocked-out soldiers will instantly drop all their equipment on the ground.
* ConcealmentEqualsCover: Since all objects can stop at least one shot. Shame aliens tend to fire on full auto.
* CoolStarship: Completing a game often requires research and construction of an "Ultimate Craft" and interrogation for the whereabouts of an alien stronghold to drive it to.
* CowTools: Aliens bases and some ships are filled with these. Some you can research, some just look appropriate.
* CrapsackWorld: [[http://forums.steampowered.com/forums/showthread.php?p=31907732 All but stated.]] An unknown, but likely large portion of the galaxy is ruled by an AlwaysChaoticEvil HiveMind. Humans ''might'' be able to destroy the local node if they become TheUnfettered -- abolish every civil liberty and article of war. And there's another, unattached (albeit slightly less advanced) node in the Gulf of Mexico. And its destruction would reduce Earth's biosphere to the algae level. And there's an entire ''planet'' of Hive Mind aliens just one dimension over. And the best weapon against all these irredeemably hostile aliens are {{Half Human Hybrid}}s with PsychicPowers... who will eventually become a permanent underclass treated like parolees from cradle to grave and not allowed to breed without permission(which tends to be withheld between invasions). In short, not only is TheVerse irrefutably hostile, but it runs on FromBadToWorse.
* CriticalEncumbranceFailure: Of the "carry items up to the soldier's Strength in weight, then take Time Unit and Stamina penalties for going overboard" type.
* CriticalExistenceFailure
** {{Averted|Trope}} with soldiers. Those lucky enough to survive alien gunfire (and that won't be many of the unarmored ones, mind you) will leak HP from "Fatal Wounds" to their various body parts until they fall unconscious and are either treated with a Medi-Kit or left to die. More often than not, it's the latter. Wounded troopers also suffer an accuracy penalty.
** Played straight with Cyberdisks. Due to how 2x2 monsters work, a stunned cyberdisk is effectively a dead cyberdisk. Actually killing it results in a [[MadeOfExplodium rather impressive boom]]. Which can also cause chain reactions, if other cyberdisks are close enough.
** Played straight in ''Terror from the Deep'' with Bio-Drones.
* CrouchAndProne: In ''UFO'' and ''TFTD'' Soldiers can crouch to improve accuracy, become a smaller target, have more cover and to allow the standing soldiers behind the crouching ones to shoot over their shoulders (though be careful, there's still a risk of hitting the guy in front of you). Soldiers automatically stand up straight when moving. ''Apocalypse'' also has a prone position.
* {{Cyborg}} / BioAugmentation: Most of the alien mooks are modified one way or another.
* DeathFromAbove:
** The Floaters and their equivalents.
** Players can also do this once they research a means of flying.
** Some Enforcer enemies will do this. In particular one floating buzz-saw thing likes to reach you and flip up to where you can't possibly get an angle on it, attacking all the while.
** The best way to use the Blaster Launcher and its cousins is typically to have the round rise to the top level of the battlescape, where it will not be obstructed, navigate to the target, and then dive straight down on top of whatever hapless alien you're blowing away. This ensures a detonation; if it doesn't hit the alien, it'll hit the ground.
* DecapitatedArmy: In most games killing the alien leader and destroying the main base he was in makes you a winner.
** One tactic for assaulting larger alien craft that have landed or been shot down involves blowing a hole to the bridge and killing the alien officers ''first''. The remaining aliens tend to panic after a few turns and drop their weapons.
** The same is unfortunately true of X-Com's forces. If the highest-ranking officer is killed, the lower ranks suffer a major morale penalty. One reason why experienced players tend to leave the Commander inside or near the dropship, where he can launch Blaster Bombs at the enemy bridge (see above).
* {{Deconstruction}}: Of children's cartoon series such as ''Franchise/GIJoe'' and ''Franchise/{{Transformers}}''. ''X-COM'' is a team of elite soldiers who wear cool-looking armor and have a fancy CoolShip that they travel the world in to save the world from goofy-looking aliens... and then suffer a relentlessly high fatality rate, crippling technological inferiority, and severe funding troubles. AnyoneCanDie, often in rather brutal ways, and 50% or higher casualty rates are common in ''successful'' missions, with failures usually resulting in [[TotalPartyKill no survivors whatsoever]]. The cool-looking armor is [[ArmorIsUseless good for little else besides appearance]]. The CoolShip costs ludicrous amount of money to lease and is completely unarmed. The goofy-looking aliens outnumber us over a thousand to one and have technology that outstrips ours to such a degree that X-COM might as well be fighting them with ''sticks''. The Man in Washington will happily cut funding at the drop of a hat, even if there's a UFO landing outside the White House. [[DarkerAndEdgier It is not a very pleasant situation.]] Ironically, after Hasbro acquired the franchise they briefly attempted to make it into a children's cartoon series, which is a rather curious decision considering ''X-COM'''s almost insanely [[RuleOfThree high casualty rate]].
* DestroyableItems: Repeat after me: No grenades or rockets in the alien engine room. Explosions can destroy any object lying on the ground, including corpses, unconscious units, and loot. Oddly enough, ammunition and other explosives are unaffected by explosions.
* DisadvantageousDisintegration: Someone got hit with the Blaster Bomb to the face in the room full of stuff MadeOfExplodium? No trace of a body and equipment.
* DisasterDominoes: When enough units are low on morale, mistakes will start piling up and feeding on each other, until recovering from a losing battle becomes almost impossible.
* DoingResearch: Any facility or weapon beyond the pitiful starting gear must be researched, which usually requires some alien material or corpse to reverse-engineer.
* DropShip: Skyranger and Triton from ''UFO'' and ''TFTD'' respectively.
* DudeWheresMyRespect: You're up against the [[RecycledInSpace outer space equivalent of]] the LegionsOfHell, including little gray men that indulge in abduction and cattle mutilation and killer crabs that give a FaceFullOfAlienWingWong. The Funding Nations don't really care about your situation, and sometimes consider siding with Aliens as a better alternative.\\\
Fridge Logic ensues: YOU are THE World Police, so many governments may not be happy about having armed foreign ships, with armor and weapons that make their best special forces seem like they're from the stone age by comparison. Also, the aliens have those nifty Psionic Powers, so they only need to look the person in charge, unleash some Brain Wash, Mind Rape or whatever... and have them as lap dogs. Sadly, the Player Character cannot do anything when a nation resigns: No amount of alienbusting activity may free a nation from alien infiltration!
* DummiedOut:
** One of the many things left unused is the "Alien Reproduction" item and research line in ''UFO Defense'' -- strange considering the resultant [[HalfHumanHybrid Half-Human Hybrids]] wind up playing a key part in ''Apocalypse''. The sequels also include other things that were ultimately left out due to time and budget constraints.
** ''Apocalypse'' also contains Procreation Parks, buildings in Megaprimus where couples go to have their children grown in artificial wombs, matching the dummied out research text of the above: "The process could be easily adapted for human reproduction".
* DungeonBypass:
** Tired of slogging through Cyberdisks and Sectoids while being panicked and mind-controlled? Breach the hull at the top floor and reach their Leader immediately with a Blaster Bomb! Other weapons can also breach the less-durable inner walls of [=UFOs=], and human buildings are all too easy to destroy. A common early-game tactic is to spam rockets and autocannon grenades on buildings that aliens might be hiding in rather than engage in costly room-to-room or building-to-building combat.
** In ''Apocalypse'', collateral destruction is a viable strategy, if you didn't mind getting stuck with the bill. Instead of scattering troops across large, multilevel facilities to hunt down aliens in dark corners, you could set fire to or blow the floors out from under their suspected hiding places and wait for the sound of their screams. [[ThereIsNoKillLikeOverkill Or just level the building with combat vehicles]].
* DynamicDifficulty:
** Regardless of the difficulty a given campaign starts at, most of the games will see fit to ramp up alien activity to correspond with consistent positive performance.
** If the player puts off going for the BigBad in ''Terror From the Deep'', alien bases will start to proliferate faster than the player can keep up with them. In other words, X-COM are fighting a losing war against superior technology. If they do not exploit their weak point by finding the BigBad, the enemy will become stronger and stronger until players have no chance of survival.
* EarthShatteringKaboom: ''Interceptor'''s Nova Bomb is designed to take out a star and everything orbiting it (a lower-level Class X-2 on the ApocalypseHow scale).
* EasyLogistics: Averted. While ammunition for conventional weapons can be bought as long as there is money, more advanced weapons require manufactured or captured ammunition to work. And then there's allocating a limited stockpile of Elerium between manufacturing and aircraft fuel.
* ElaborateUndergroundBase:
** A necessity due to X-COM's covert nature, often leading to AllYourBaseAreBelongToUs should the aliens stumble upon it (hopefully there is more than one base built by the time an Alien Retaliation fleet comes calling).
** Aliens have underground installations which need to be broken into to kidnap high-ranking officers for interrogation to complete the game.
%%** ''Enforcer'' does this in a few levels.
* EliteMooks:
** Alien Squad Leaders in general.
** Enforcer bosses, which are usually giant versions of other mooks like Reapers or Chryssalids, with special attacks.
* EmotionBomb: "Panic Unit", the easier use of psi powers.
* EnemyDetectingRadar: Motion Trackers, though they can only detect units that moved within the last turn. So always check the corners in case something's lurking.
* EnemyScan: Mind probes, which allow the player to view an enemy's stats.
* TheEnemyWeaponsAreBetter: Damage-wise at least.
* EnergyWeapon: Laser and Plasma weapons.
* EncyclopediaExposita: The [=UFOpaedia=] in its various forms across the generations. Even more so [[http://www.ufopaedia.org the fansite of the same name]]. And [[http://ufopedia.csignal.org/ there is more or less an online version]] of the in-game [=UFOpadias=].
* ExclusiveEnemyEquipment: The few thing that your technicians cannot reproduce, including the alien power sources.
* ExplodingBarrels:
** Fuel drums in your bases, gas pumps in Terror Missions, and certain UFO components all explode when shot. Frustratingly, so do Elerium pods exposed to explosions.
** In ''TFTD'', the normal skirmishes (USO Recovery) sometimes have what seems to be oil pumps. Also, apparently sunken aircraft's engines are MadeOfExplodium.
* ExposedExtraterrestrials: Sec[=/=]Aquatoids, the rest of non-terrorist ''TFTD'' aliens and all of ''Apocalypse'' ones.
* FaceFullOfAlienWingWong:
** Chryssalids don't necessarily use the face, but they do reproduce by forcibly implanting eggs in human hosts that burst upon death to reveal a new-born Chryssalid ready to fight..
** Tentaculats and Brainsuckers reproduce in similar fashion in ''Terror from the Deep'' and ''Apocalypse'', respectively.
** Brainsuckers attach onto people's heads and empty their innards down their throats.
* FakeDifficulty: ''Terror From the Deep'' is probably the worst offender, but the game balance would not suffer if the standard rifle from ''UFO Defence'' were capable of reliably hitting anything a distance greater than it could be thrown.
* FanRemake / FanSequel: Free ones include and are not limited to ''[[http://ufo2000.sourceforge.net/ UFO2000]]'', ''[[http://ufoai.sourceforge.net/ UFO: Alien Invasion]]'', ''[[http://openxcom.ninex.info/ OpenXcom]]'', and ''[[http://ufotts.ninex.info/ UFO: The Two Sides]]''. Commercial ones include and are not limited to the ''VideoGame/UFOAfterblank'' series, the ''[[http://ufo.ufo-extraterrestrials.com/ UFO: Extraterrestrials]]'' [[http://www.ufo2extraterrestrials.com/ series]], and ''VideoGame/{{Xenonauts}}''.
* FlamingSkulls: Skull-shaped explosions.
* FriendlyFireProof: Averted, and with the average accuracy of X-COM soldiers, frequently painful.
* FrickinLaserBeams: The Laser weapons obviously. In the first ''X-COM'', they're extremely useful throughout the early and mid-game and retain effectiveness in the late game, as they use no ammunition. Laser pistols also have the added advantage of an very low TU cost to their autofire, making them ideal for room-to-room combat against anything short of Mutons and Snakemen missions with Chryssalids. Sectopods, the Ethereal's terror units, are more vulnerable to laser beams than plasma.
* GatlingGood: The Autocannons and their successors. Regular and incendiary ammo rapidly become obsolete, but high explosive rounds remain viable throughout the game. Being able to saturate an area with high-explosive bullets never ceases being effective or awesome.
* GenreSavvy: You're going to need to be in order to win. Just remember; if they don't make the death scream, they aren't dead.
* GenreShift: What happened to every single ''X-COM'' game after the third one.
* GeoEffects: On the Strategic scale, where you land determines what kind of terrain it will be in the battlefield. For example in ''TFTD'' landing in seas around Europe makes it very likely that the mission will take place among the UnderwaterRuins, and in the very deep areas it's dark as in the night mission even during the day.
* GiantMook:
** Many terrorist aliens.
** The Megaspawn from ''Apocalypse''.
** ''Enforcer'' bosses.
** ''UFO Defense'' gives us the Reapers and Sectopods, the former a glorified alien attack dog and the latter a heavy assault mecha. Terror from the Deep has the Xarquid, a giant nautilus, the Triscene, a [[EverythingsBetterWithDinosaurs dinosaur with Sonic Cannons]], and the Hallucinoid, a prehistoric jellyfish with [[AnIcePerson chemical freezing agents]].
* GiantEnemyCrab
** The Chryssalids in ''UFO Defense''.
** The Lobstermen in ''Terror from the Deep''.
* GlobalCurrency: Everything bought and sold in the main games is apparently done so in U.S. dollars.
* GloriousMotherRussia: And they're surprisingly [[BadassNormal badass]], too -- Russia is the only nation in the game that can never be subjugated by the aliens, no matter how bad things get. They'll fight until the last man falls.
* GrenadeHotPotato: With a little coordination and luck, a soldier in the back can prime and pass a grenade to the front.
* TheGreys: But of course.
* GridInventory: Multiple grids throughout the body and uniform (and multiple Time Unit costs for movement of items from location to location) make a refreshing take on the InventoryManagementPuzzle. The ''true'' InventoryManagementPuzzle (at least in the first game or two) was deciding what 80 pieces of gear to bring along on a mission. A fully loaded Avenger/Leviathan (holding 26 soldiers) could consume 52 of those slots just giving each soldier a gun and its ammunition. And that's without bringing extra ammo for reloading.
* GunsAkimbo: Doing this in ''UFO'' and ''TFTD'' only gave you another weapon to fire from with penalties. Expended in ''Apocalypse''.
* HighTurnoverRate: If you're not good enough or just plain unlucky, most X-COM soldiers will not last more than a month or two, leading to this.
* HiveMind
** The aliens in ''UFO Defense'' take orders from one, another in ''Terror from the Deep'' tries to tell a CosmicHorrorStory while it's at it, and the Biomass in the ''UFO'' games is a weapon designed to [[GeniusLoci turn a planet into one]].
** The true threat in ''X-COM Apocalypse'' are actually colonies of microscopic organisms that are sentient in groups. Every alien life form you encounter in that game is merely one they've managed to take control of and manipulate to their own ends; the "brainsucker" life form that turns your comrades against you just injects their brains with an overload of micronoids. Late in the game, some [=UFOs=] try to take control of buildings and organizations by directly sprinkling lots of micronoids onto the building in question to influence the minds of those within.
* HoldTheLine: The objective of any Base Defense mission. Can literally turn into Hold The Line if you choose to adopt such tactics, although in some cases it's not so much line-holding as shooting fish in a barrel with a BFG. This probably only applies if your base is attacked early in the game where aliens' psi attacks turn it into a nightmare. But, once you screened your recruits (and sack the weak minded ones) and researched alien weapons, even if you have poorly designed base, a defense mission is just a shooting gallery.
* HollywoodHealing: Averted, wounded soldiers have to stay on lengthy medical leaves, with the most serious cases taking months.
* {{Hovertank}}: With your choice of Fusion Bomb launcher or Plasma cannon. Arguably, the Cyberdisc can be considered one of these sans turret.
* HumanoidAliens: Most of the weapon-using aliens you face are humanoid in appearance, and in some cases were human before being genetically modified. The rest, with few exceptions, are non-humanoid terrorists units that support their masters.
* HumanResources: Many research reports state that the aliens "harvest" humans for various purposes, from research to organ extraction.
* IdiosyncraticDifficultyLevels:
** [[EasierThanEasy Beginner]], Experienced, Veteran, Genius, [[HarderThanHard Superhuman]].
** ''Apocalypse'' is a bit simpler: [[EasierThanEasy Novice]], Easy, Medium, Hard, [[HarderThanHard Superhuman]].
* ImmuneToBullets: Because of how armour works and damage is rolled, certain enemies are actually immune to standard rifle or pistol rounds. Some are even highly resistant to otherwise powerful alien weapons (hello, Sectopods and Lobstermen).
* ImportedAlienPhlebotinum: About 75-90% of gameplay revolves around the acquisition, understanding, and implementation of cool alien toys. Or in the RPG terms: Kill them, take their stuff, reverse-engineer it, Repeat. Reversed in ''Apocalypse'': when you sell some of your stuff to a MegaCorp that's been infiltrated by aliens, the aliens will import ''your'' phlebotinum.
* Instant180DegreeTurn: Averted. Turning costs Time Units, though it will not trigger alien reaction fire. Sometimes, troopers who have to turn to face the enemy wind up without sufficient [=TUs=] to take the shot.
* InstantSedation: The enemy is stunned if the stun damage exceeds the normal health. Thus, it is usually played straight for weaker aliens and those with weakness to stun damage, unless you are unlucky, but generally averted for stronger ones.
* IsometricProjection: The perspective used for Battlescape.
* ItemCrafting: Manufacturing the reverse-engineered alien technology and our own creations.
* ItsUpToYou:
** Since all of the world governments have tried and failed to handle the aliens, it's up to X-COM to get things done. Although it wouldn't hurt if the local governments lent a hand during Terror Missions.
** ''Terror from the Deep'': X-COM was disbanded after the end of First Alien War, and the world governments don't have the advanced technologies they developed to confront the new alien threat. Once again, it all comes down to X-COM.
* IWantThemAlive: When researching Alien Origins and latter item down the paths, the reports state that you need to capture higher ranked aliens.
* JetPack: Your troops get these during the late game (or from the start in case of ''Apocalypse''). The flight-capable aliens (except for the psychic aliens who use their powers to fly) usually have these... [[HollywoodCyborg built]] [[BodyHorror in]].
* KillItWithFire: Burning the zombies (with incendiary ammunition) will also kill the chryssalid\tentaculat without it popping out.
* LawEnforcementInc: After the Second Alien War of ''TFTD'', X-COM becomes this to avoid underfunding. X-COM's more ambitious cousin Marsec started out as a replacement for the former in guarding the martian colonies so that they could concentrate on potential alien threats, but soon becomes a para-military corporation with a ruthless reputation. Megapol from ''Apocalypse'', in addition to being a police force, also operates other 24-hour services, the fire fighters and the hospitals.
* LensmanArmsRace: As X-COM improves their arsenal and knowledge via research and reverse-engineering, the aliens will start sending bigger [=UFOs=] with larger groups of better-trained soldiers wielding bigger guns with nastier support monsters, after which X-COM will improve their arsenal and knowledge via researching and reverse-engineering of anything this new wave had on their dead bodies. Rinse and repeat.
* LightningBruiser: Mutons and especially the Lobstermen.
* MadeOfExplodium: Lots of objects on the battlescape will explode when shot. Special mention goes to the Cyberdisc and its successor, the Bio-Drone. When they die, they explode with stunning force, causing collateral damage. This can help for better or for worse, depending on who the explosion kills.
* MarathonLevel: The two-parter missions.
* MarketBasedTitle: The original game was called ''UFO: Enemy Unknown''.
* UsefulNotes/{{Mars}}: Alien HQ in ''UFO'', colonized by humans after ''TFTD'' and is the humanity's main source of elerium-115.
* MechaMooks: Various Terrorist aliens, most of them being mini-FlyingSaucers.
* MeetTheNewBoss: Pretty much every knew alien threat faced in the original series, when it comes down to how they act and what they're goals, are essentially same product new packaging of the original aliens.
* MercyMode: Having a particularly bad month performance-wise or worse, a string of bad months, will make the game take pity on you by making the "X-COM agents discover the Alien Base". Technically possible even if you're doing fine, but much more common when you're doing bad.
* MindControl: The ''harder'', but much more useful, use of psi powers.
* MindProbe: A handy tool for either side to [[EnemyScan gather information on the other]]. Best used for determining how close an enemy is to collapsing from stunning, or whether that alien right there is a Soldier/Medic (Mook), Engineer (useful for research), or a Commander (Boss, crucial to capture in the late game). Becomes obsolete once you get high-psi strength units equipped with psi-amps, which can take total control of an enemy and allow you to see its stats at any time for the remainder of the turn.
* MonsterCompendium: Via [=UFOpaedia=].
* MoneyForNothing: UFO parts and corpses sell quite well and X-COM itself can self-finance through arms manufacturing.
* MoraleMechanic: Casualties and psychic attacks can cause your soldiers to panic, moving and shooting at random. The presence of a high-ranking officer can reduce morale loss from casualties, but an officer's death has a larger effect on morale.
* MultinationalTeam: You recruit from around the world. However, recruit names are exclusively drawn from Russian, French, German, Japanese, and British/American pools (the last two being a little difficult to distinguish). ''TFTD'' adds Spanish and Italian pools.
* MyBrainIsBig:
** The Sectoids and Aquatoids are small, impish humanoids with enormous heads. Certain variants have psychic powers.
** Ethereals aren't exactly under-endowed in the grey matter department. Their design in ''Interceptor'' has a brain large enough to apparently need air-cooling.
** The Alien Brain, which is precisely what it sounds like, and the final boss of ''UFO'', is very big indeed.
* MythologyGag: Marsec's first appearance was in ''Laser Squad'', the spiritual predecessor to ''X-COM''.
* NintendoHard: ''X-COM: UFO Defense'' and ''X-COM: Terror From the Deep'' are infamously difficult, as the game system is very complex, and almost every random variable in the game has a high variance in either direction. No punches are pulled early in the game, with most every enemy type capable of appearing in missions from the get-go, and at no point does a soldier ever become safe from being instantly killed by a stray shot. Even the safest move is a gamble for both sides, although the aliens aren't bothered by losses. In ''Terror From the Deep'', because the (then unknown) bug in ''UFO Defense'' locking the difficulty to Beginner prompted the fans to complain about it being too easy, the developers made the Beginner setting of ''Terror from the Deep'' as hard as the Superhuman of ''UFO''. There's a [[UrbanLegendOfZelda common rumour]] that ''TFTD'' had the original's bug backwards, locking difficulty to Superhuman. It doesn't; it's just a ''lot'' harder.
** The way the game generates missions and is also utterly unforgiving. A far cry from today's games that insist on giving you a shallow learning curves and holding your hand through the most simple of tasks, X-COM has no scruples against sending you on a terror mission with nearly bullet-proof Cyberdisks or Chryssalids in the early game, or positioning aliens in a half-circle around your Skyranger's disembarking ramp, ready to mow down whoever tries to disembark. Know when to fight and when to attempt a frantic TacticalWithdrawal.
** Since they couldn't replicate the original Alien AI with the modern code, ''[=OpenXcom=]'' developers had to make it smarter than in the original just so it would not be curbstomped, and there is a Sneaky AI option that makes it even harder.
* NonLethalWarfare: Stun Rods (melee spears), Stun Bomb Launchers (basically a grenade launcher that fires stun bombs) and their successors. The only other way to capture aliens is to make them pass from pain from normal weaponry.
* NothingIsScarier: Night missions can be exceptionally creepy, especially in ''Terror from the Deep''. It's pitch dark and there are aliens packing enough firepower to drop soldiers in one hit, and furthermore, since it's so dark, you don't know where they could be hiding. It can get even worse if you're on a Terror Mission with Snakemen, and you know that there are Chryssalids just waiting for you to screw up.
* OmnidisciplinaryScientist: The same staff of scientists does everything from reverse-engineering captured weapons to designing new aircraft to interrogating prisoners. Although considering that you NEED a lot of them to have a decent research progress, it could be handwaved that, say, a research on Plasma Weapons is led by the specialists in the field with everyone else following instructions. In ''Apocalypse'' we got three types of scientists: Engineers, Quantum physics and Biologists.
* OneHitPointWonder: Your units actually have a life bar but, until they gain lots of experience or get some BETTER armor, they might as well have one hit point. This is only averted at the start of the game by the very rare occasion of a soldier surviving a Plasma/Sonic shot in their starting armor due to a low damage roll.
* OneHitKO:
** Chryssalid melee attack. The same goes for its successors.
** Soldiers can be easily killed in one hit from a Plasma gun, even if he is wearing a Power Suit, if the damage roll is high enough.
** Vibro Blades in ''Terror from the Deep'' are capable of killing most aliens in one or two hits, including the [[ImplacableMan Implacable Lobstermen]].
* OneStatToRuleThemAll: Psionic Strength in the first game (and its cousin MC Strength in the second) is the only stat that cannot be trained and it determines both resistance to alien mind control and the soldier's ability to control aliens. Actively using psionic abilities provides experience for all but three other stats.
* OneSteveLimit: Averted due to limited names pool. With only a relatively small amount of names to pick from when a new soldier is randomized, it's not uncommon to see several share a name.
* OnlyAFleshWound: Averted. Arm and leg wounds will greatly reduce a soldier's fighting ability, just like head and torso wounds. And that's on top of bleeding to death.
* OrganicTechnology: All sorts of purpose-bred aliens in the first two games, and practically every aspect of the alien threat in ''Apocalypse''.
* OrganizationWithUnlimitedFunding: ZigZagged; the Funding Nations[=/=]Senate are huge cheapskates, but once enough engineering facilities get going and start cranking out weapons to sell, X-COM can effectively go rogue. However, the better the player does at protecting a given nation, the more they'll increase funding. As the game only ends when the player is in the red for too long or if all nations sign non-aggression pacts with the aliens, it's possible to keep protecting a few nations at a time and building up their funding over time to replace the nations that drop out. In a sufficiently long game, it's not uncommon to have the US providing 14-20 million dollars a month to X-COM... as the only nation still funding it.
* PacifistRun:
** ''UFO'' and ''Terror From the Deep'': To progress, the player needs to capture enemy aliens alive to interrogate or inspect them. Cue the player loading up on stun rods and other nonlethal equipment (depending on which installment in the franchise) so they can capture most or all aliens alive in one mission (and then get back to the slaughter once research is complete).
** In ''Apocalypse'' if a player raids an organization without killing anybody or damaging any property, the relations aren't penalized, despite the stolen loot. Plus ''Apocalypse'' has stun grenades, ranged stunners and mind control available from the start. Many players raid Marsec for all the cool weapons not available from the start or in short supply. There are also [[PsychicBlockDefense Mind Shields]], which were DummiedOut and cannot be bought.
* PeopleJars: You will find these in the Alien bases. Unfortunately, they contain what left of their occupants.
* PlayerHeadquarters: Though there is no HQ in the strict sense once you have multiple bases, the cost and time associated with building bases and the subsequent maintenance fees will make the first starter (and already developed) base your main base of operations. Which is ''bad'', because it's the most likely base to be attacked, and has an ''extremely'' difficult-to-defend layout.
* PlayerMooks: Soldiers, available in unlimited quantity and variable quality until trained in battle.
* PoweredArmor: Later armor suits in ''UFO Defense'' (and the whole lot of 'em in ''Apocalypse'') use Elerium-115 to power shielding, muscle enhancers, and the occasional flight module. Flight and protection are the only benefits; armour has no bearing on a soldier's strength or other stats (the muscle enhancers are just [[InformedAbility fluff that indicates a lack of weight-related decrease in soldier performance]]).
* PowerCrutch: The [=PsiAmps=] and their variations that enable X-COM soldiers to use psychic powers.
* ProperlyParanoid: Anyone who takes ''great'' care while handling Terror Missions. If you aren't covering all the angles, you're just TemptingFate.
* PsychicAssistedSuicide: Aliens won't shoot themselves, but they can be made to drop primed grenades at their own feet or drop their guns (only in the first game).
* PsychicPowers:
** Of the telepathic kind, making victims panic (or go berserk) to controlling them.
** The second game [[CallAHitPointASmeerp renamed Psi]] to Molecular Control, which [[AndIMustScream is suitably more horror-inducing]].
* PsychicBlockDefense:
** Androids in ''Apocalypse'' cannot be controlled at all, on account of being robots.
** The protagonist of ''Enforcer'' is [[implied]] immune to mind control, considering the situations the player gets into.
** [=HWPs=] and [=SWSs=] in ''UFO'' and ''Terror From the Deep''. Being unmanned vehicles, mind-affecting powers do not affect them.
** One of the novelizations has a guy who happens to have a Psi Strength of zero. [[CursedWithAwesome It makes him unable to use psi powers at all but they don't work on him either.]]
** There are only two aliens in ''Apocalypse'' that are easily affected by Psionics, them being the Anthropod and Skeletoid. All other creatures are resistant to Psionic attacks because they are either less intelligent (which somehow boosts resistance), in the case of Multiworms or Spitters, or possess Psionic abilities themselves, like the Micronoid Aggregate and Psimorph.
** ''Apocalypse'' has Mind Shield -- an item increasing psionic defense when used in the battlescape. Due to a bug the increase is permanent and cumulative, thus any operative can become invulnerable. The item was supposed to be removed completely from the game, but due to another oversight may be encountered during missions in Marsec buildings. Fans have come with an in-universe explanation that permanency and cumulation were unexpected side effects, Marsec found them disastrous for their business -- killing the market both for Mind Shields and Mind Benders -- and recalled all unsold items shortly before the game start.
* PurelyAestheticGender: Recruits can be either male or female. This has absolutely no effect on starting stats or stat growth. All it does is paste a slightly different head on the sprite and give ladies a different scream when they die. (In the second game, at least. In the first game, all aliens and all soldiers sound the same when they die.)
* RandomlyGeneratedLevels: Using an algorithm to procedurally construct levels from "tiles".
* RandomNumberGod: In addition to accuracy rolls, all weapons and explosives do a random amount of damage. Guns roll from zero to double their listed damage, while explosives roll from 50% to 150%. A lucky soldier can survive multiple bursts from heavy weapons, whereas an unlucky one dies from an ally's missed shot.
* {{Realpolitik}}: There is a little bit of this in how the [[http://www.ufopaedia.org/index.php?title=Country_Funding_(EU)#Monthly_Funding_Change Council of Funding Nations reevaluates its funding of X-COM]]. Ex: X-COM performed excellently in March except for that time where the few particularly large [=UFOs=] flied over Country A unopposed causing trouble, giving Country A enough reason to reduce funding, but since the Council's majority opinion of X-COM is overwhelmingly positive that month, Country A has no choice but to stay quiet, not increasing the funding as their only means of protest. The only exceptions are when X-COM performs extremely badly at the complaining Country or if Alien infiltrated it. The reverse is also true, where very few will dare to increase funding when the Council is very unsatisfied with X-COM.
* RealTimeStrategy: The Geoscape[=/=]Cityscape screen is real-time, pausing for events like delivery of ordered items, an alien sighting, or the arrival of troops at a mission site.
* RedEyesTakeWarning:
** The Reapers in ''UFO''.
** The Aquatoids in ''Terror from the Deep''.
* RedShirtArmy: At the start of the game, the player's soldiers are all ridiculously fragile, and as likely to die as to inflict any damage at all.
* RidiculouslyFastConstruction: Building a base gives you an instant lift. Other than that, the game avert this.
* {{Roboteching}}: Blaster Launchers and their equivalents with their diabolical waypoint-based targeting system.
* RPGElements: Soldier stats in all games.
* SaveScumming: A common strategy, unless you think this is cheating or dishonorable -- it is possible to win the game with 0 casualties. The game designers do make a token effort to discourage it; you can't load a saved game during a combat mission, although you can ''save'' all you want in the middle of a mission.
* ScaryDogmaticAliens: The bad endings classifies the aliens as the Conqueror type, who attack Earth for its resources.
* SecretWar: All of them start as one, but according to the manuals all of them become open secrets later.
* SensorSuspense: Motion detectors: a good way to avoid becoming Cannon Fodder when facing alien weapons, but since you don't know whether the blip is from alien or civilian and on which floor, dealing with the results can be... interesting. Two words: Hidden Movement.
* SeeingThroughAnothersEyes: Through mind-control.
* SelfImposedChallenge:
** Probably the greatest fan-mod undertaking for ''UFO Defense'' is fixing a bug that locked the starting difficulty of the game at "Beginner".
** There are a bunch of challenges listed with one of the well-known editing utilities, including things like refusing to research any new tech (thus making the game technically unwinnable, but few get to such a point since it's damn difficult without) and not killing any aliens (winning is still possible since they can be stunned).
** Zero loss run: No X-COM agents lost, no civillians lost. Hope you have a few months spare.
** [[http://ufopaedia.org/index.php?title=User_talk:Arrow_Quivershaft#1_Mission_X-COM One Mission X-COM]]: It's possible to complete the game after completing only one mission: a well-executed UFO Ground Assault on a battleship can yield all the alien prisoners and raw materials needed to research Cydonia and complete the game. The difficulty is in pleasing the Funding Nations (no terror missions or alien base assaults allowed), and in keeping away from bankruptancy.
* [[CapcomSequelStagnation Sequel Stagnation]]: Averted. After ''Terror from the Deep'', which was basically [[MissionPackSequel an underwater rehash of the first episode]], new elements and even {{Genre Shift}}s were introduced -- unfortunately, they didn't result in good games.
* ShinyLookingSpaceships: [=UFOs=] of the first game. X-COM fighters in ''Interceptor''.
* SmokeShield: Caused by specialized smoke grenades and explosive terrain features.
* SortingAlgorithmOfEvil:
** In ''UFO Defense'' it's Sectoids and Floaters and early Snakemen if you're unlucky > Snakemen if you're lucky > Mutons > Ethereal.
** In ''TFTD'' it's Aquatoids and Gillmen > Tasoth and Lobstermen.
** Also lampshaded in Guava Moment's ''Apocalypse'' LP.
* SpiritualSuccessor: Many. The ''UFO: Afterblank'' trilogy to name one.
* SplashDamageAbuse: The great vulnerability of ground-based vehicles in ''Apocalypse'', including the tank. Also, most units take more damage from explosives that go off at their feet because of lower under armour.
* SprintMeter: While Energy[=/=]Stamina is used for everything, its biggest consumer is moving around. It only partially regenerates at the start of each turn (Less when carrying heavy gear), so if an X-COM soldier moves a lot without stopping (such as when reserving Time Units for an Reaction Shot), they eventually will be too tired to move more than a few tiles.
* StrongFleshWeakSteel: With experience and armour, soldiers eventually gain much better stats than tanks and become more durable. However, tanks do retain a few advantages in that they cannot be stunned, mind controlled, or bleed out.
** Inverted in ''Enforcer'', where you will chew through Sectoids, Snakemen, and Chryssalids with one shot each while their feeble attempts to harm you bounce off your armored hull. [[LetsPlay It will give your]] [[ShoutOut positronic brain much amusement.]]
* StatGrinding: How the soldiers improve their stats.
* StockSoundEffects: Some of the sounds that occur while navigating the menus of the original ''X-COM'' have spread far and wide across various media, recently being used in the title character's HUD in the big-budget movie ''Film/IronMan''!
* StrayShotsStrikeNothing: Averted. With the exception of map borders where the shots disappear, all missed projectiles will continue to fly until they hit something: walls, explosive materials, civilians, X-COM operatives, aliens, etc.
* StunGuns: From Stun Rods to Stun Bombs, a variety of nonlethal arms gradually come into X-COM's possession and employ for the capture of necessary live aliens.
* SubsystemDamage: Head, Body, Separate Arms and Legs, with penalties one soldier's efficiency depending on injuries. Assuming the soldier in question survived the first shot.
* SuperSoldier:
** Mutons and Lobstermen. Your soldiers will become this if they are lucky enough.
** ''Enforcer'''s protagonist, who racks up over a hundred dead aliens a mission, and sometimes as many as four hundred fifty.
* TacticalWithdrawal: When the situation gets hopeless on the battlefield there is a option of retreating. The good side of this is that your soldiers live to see another day, the sponsors will be less angry than with total defeat and thus complete loss of expensive equipment, and that you could yoink some alien artifacts. Just be aware that everything that was outside the X-COM transport will be lost and MIA.
* TakeCover: Very important, given the computer's cheating tendencies and the power of alien weapons. Unfortunately, most forms of cover can be destroyed.
* TankGoodness:
** [=HWPs=] are a refreshing alternative to the hopeless rookies in the early game, at least in ''Enemy Unknown''.
** ''Terror From the Deep'': [=SWSes=] do make good scouts if you don't like sacrificing rookies for that. Once you get a Sonic Displacer, you will like it for its ability to float, get 200 shots clip which gets reloaded for free every missions, and ultimately, [=SWSes=] can't be harmed by tentaculats. If you already have the bigger ships, you will always want to bring one (or two) on every missions.
* TeamTitle: Subtitles aside, the series is named for the anti-alien unit commanded by the player.
* TechnologyLevels: Human Starter Tech < Advanced Human Tech < Alien Tech = Alien-based Human Tech.
* TechTree: While almost all physical alien artifacts can be researched as soon as you recover them, several conceptual lines of research require either the interrogation of live aliens or a series of prerequisites.
* TelepathicSpacemen: Ethereals, experienced Sectoids, their underwater cousins Aquatoids, Gill Men commanders, Tasoths, Psilords in ''Interceptor'', the list goes on...
* TerrorHero: Your psionic soldiers can use their psi abilities on the aliens to cause them to panic, making them easier to kill or capture.
* TimeKeepsOnSlipping: The various incarnations of the Geoscape allow you to pass the time by anywhere from 1 second per second (slowest setting in ''Apocalypse'') to 1 day per second (fastest setting in ''UFO Defense'' and ''Terror from the Deep'') while you're waiting for the next alien sighting.
* [[TotalPartyKill Total Squad Kill]]: Get ready to get used to these because it'll happen a lot throughout your playthrough.
* TrickBomb: Smoke and Proximity Grenades. The former are for obscuring vision, the latter are throwable mines.
* TurnBasedTactics: The Squad-Level type during the battlescape.
* {{Unobtainium}}
** Elerium-115, in spades.
** [[ElementsDoNotWorkThatWay Ununpentium has long been theorized to work that way, but currently doesn't.]]
** Also, ''Terror from the Deep'''s Zrbite.
* UnusableEnemyEquipment: Which will become usable after research.
* UnfriendlyFire and/or TheUriahGambit: Note that the [[AfterActionReport post-mission analysis]] does not have a spot for "X-COM Operatives Killed by X-COM Operatives." Your use of this oversight to justify friendly fire or the immediate court-martial of an alien-controlled operative will practically be a given.
* UrbanWarfare: Terror missions. Most of ''Apocalypse''.
* VideogameCaringPotential: Just ''try'' not to get attatched to your soldiers who got promoted to Sergeant or above rank.
* VideogameCrueltyPotential: What happens if you make a bunch of rookies designed ''explicitly'' to be cannon fodder and/or scouts (yes, the two jobs overlap frequently). This makes for some delicious BlackComedy as you can name the unfortunate saps such names like "Dead Man Walking" and "Cannon Fodder".
* VendorTrash: All those alien corpses, spare UFO/USO compoments, and ammunition? Yeah, a good deal of that gets sold off to finance the organization. Even a relatively small UFO captured intact can net nearly a million dollars depending on how much loot you hold on to. Alien corpses are often extremely convenient for research, but supply exceeds all possible demand, so selling them is a non-trivial but disturbing source of income. (A strategy guide suggests that they make for excellent sushi. And a certain fan wiki suggests that bases frequently fighting Lobstermen requisition above-average amounts of butter.) Well, they do the same to us in the first place. They deserve every bit of what the scientists and chefs do to them.
* TheVeryDefinitelyFinalDungeon: The various Aliens' [=HQs=]. Failing the missions results in instant gameover.
* VeteranUnit: The Lucky Soldiers who survived enough battles and killed enough aliens.
* VichyEarth: Earth slowly becomes this over the course of the game if you do badly or take too long, as more and more countries submit to the aliens. Apart from GloriousMotherRussia. Fan studies of long-term games have concluded that out of the Council of Funding Nations, Russia will ''never'' be infiltrated by aliens. The newspapers will instead probably show Sectoid ambassadors' corpses nailed to the gates of Kremlin.
* TheWarOfEarthlyAggression: Mentioned in the backstories of ''Interceptor'' and ''Apocalypse''.
* WeHaveReserves
** Rookies make good scouts.
** The game mechanics actually encourages a WeHaveReserves mentality. If you feed ''TFTD'' rookies to keep it happy, the game can be quite manageable... but if you've truly mastered your tactics and so almost never lose a man, you're ''screwed''.
** This mentality is sort of averted in ''Apocalypse''. If you're good enough to consistently keep your squads alive in the early game, the difficulty ''will'' scale up quickly, but the aliens will be getting better weapons before better units (and an Anthropod holding a Devastator Cannon is still just an Anthropod), so if you really are that good, you can stay on top of the game all the time.
** The death of a rookie is less damaging to a squad's morale than the death of a higher ranking agent. And if you're [[VideogameCrueltyPotential feeling particularly cold]], the aversion of EasyLogistics makes it a lot easier, and cheaper, to send rookies in to combat with a bare minimum of equipment so that higher ranking and more skilled agents can get the good stuff.
* WeSellEverything: Played straight for most of the series. It makes one wonder what the people do with the alien artifacts you sell, especially the ''corpses''.
* WithThisHerring
** With this bunch of folks who would have failed the physical for any self-respecting military and have the reflexes of a dead fish, you must save the world...
** The standard-issue X-COM rifle is supposedly based on the best traits of a variety of human firearms, combined into one package perfect for your work. Unfortunately, it was built by the lowest possible bidder.
** Played even straighter in ''Terror from the Deep.'' The standard equipment you get is ''worse'' than their UFO counterparts, and about half of them only work underwater. In this game, it reaches Wallbanger levels: At first you're limited to your subs, darts guns and some harpoon launchers, since nothing you had before would work underwater. Then, you can have to fight the aliens on dry land, due a crash, a landing or a terror mission. Why, oh why you must still carry dart guns, instead of old Earth-made laser weapons, and wear a heavy submariner suit when you could have crates and crates full or armor from the first alien war? Heck, local millitias should be better armed and prepared than you at this point!
** Despite that fact that "Starlight" night vision scopes and binoculars have been available since at least the Vietnam War and passive infrared night vision has been around since the 1980s... you're reduced to throwing flares.
** In ''Apocalypse'' your starting weapons are a bit better, and Earth has some weapons on a par with alien ones. Money are short, but you can raid Cult of Sirius. But you can only buy what is available in the city stores, which get new shipments only on Mondays. You cannot buy best weapons and equipment (flying armor, mini-launcher, plasma pistol, plasma sword, powerful vehicle computer and engines, heaviest vehicles) until the start of the second or third week. And if you try to ZergRush [=UFOs=] with rocket and plasma hoverbikes, Megapol, Marsec and Solmine quickly run out of missiles and elerium to sell to you.
* WhoYouGonnaCall: Type 3, the Organization.
* YetAnotherStupidDeath: Players who are unlucky, forget to take precautions, or just play poorly, will get lots of these. Even a seasoned player will have some of these from time to time.
* YouAllLookFamiliar: A good number of soldiers will have similar appearances on the inventory screen. Played Straight in missions. On the map they virtually all look alike. The guys all have [[Franchise/StreetFighter Guile haircuts]] and the women have ponytails. Ditto for civilians.
* YouRequireMoreVespeneGas:
** Money, required to fund everything you do. Your official source is from the funding nations, who increase or decrease based on your performance. Your unofficial source is selling your spoils of war and manufactured goods on the black market.
** Alien Equipment, with the most important of them being the Elerium, the alien power source that cannot be reproduced, only salvaged.
** Hangars, Living Quarters and General Stores for increasing the Craft, Population and Item caps, respectfully. Similarly, having more Laboratories and Workshops will allows more Scientists and Technicians to be able to work.
* ZergRush
** The only way to take a small alien ship in ''Enemy Unknown'' is to keep pushing soldiers through the door until you kill everyone inside. This will almost certainly cost you several troops.
** On a larger scale, X-COM typically has extremely high casualty rates in all three games and [[ZergRush Zerg Rushing]] strategically to replace lost soldiers and interceptors is the only way to keep your head above water.
** On the other hand, most of the aliens will leave their ship and actively hunt for you starting with the 21st turn. Camping by their UFO doors lets you snipe them as they poke their heads out.
** It's also possible, with careful Time Unit management and a sizeable slice of luck, for one or two of your team members to toss in a grenade and get out of the line of fire in a single turn.
** One-robot Zerg Rush is probably the best way to play ''Enforcer'', as it lets you get those research points.
** You can take down the largest [=UFOs=] in ''Apocalypse'' using the smallest flyer available, hoverbikes. They're practically impossible to hit with the right settings. [[StarWars Worked for the Rebels.]]
[[/folder]]

[[folder:UFO Defense[=/=]Enemy Unknown (1993)]]
* AbusivePrecursors: [[spoiler:The Alien Brain in ''UFO Defense'' claims that the aliens created humanity.]]
* AchillesHeel
** A misprint in the EncyclopediaExposita entry fro Sectopods means their primary weapon counts as a laser attack (which they're weak to) rather than a plasma attack. Combine that with the fact that 2x2 enemies have to be mind-controlled one block at a time and that blocks controlled by opposing sides can fire on one another... Controlling part of a Cyberdisk or Sectopod is, in fact, the easiest way to destroy one.
** Flying suits will bring aliens with melee-only attack, including the fearsome Chryssalids, at your mercy.
* TheAestheticsOfTechnology: The alien technology is sleek, shiny and spotless.
* AlienLunch: The "Alien Food" is a nutrient soup made from body parts extracted from cattle and humans. You can even sell it on the market.
* AllDesertsHaveCacti: Any desert you visit in ''UFO Defense'' has Cacti. Even the deep Sahara or the Rub al Khali. Even the desert in ''Enforcer'' has cacti.
* {{Animesque}}: The intro of the ''UFO Defense'' is proto-animesque, and the demo's list of features mentions a "popular 'Manga' look and feel to graphics." The background images for base functions and Hidden Movement retain the art style of the intro. Otherwise, the graphics are about as realistic as can be expected from a game of its age.
* ApocalypseHow -- Planetary: If you fail to stop the aliens, they take over the world, destroy human civilization completely, and humanity ends up as a race of semi-intelligent mutants at best. Also, the sky burns.
* ArmlessBiped: The Reapers.
* AstralFinale: You fly to Mars to assault the Aliens' HQ, the Cydonia base.
* BigNo: A rare written example. [[spoiler:The alien brain tries to talk you out of killing it via a computer screen at its base. The screen displays an enormous-font "NO" as the brain is incinerated.]]
* BriefcaseFullOfMoney: A picture of a high-ranking X-COM agent with one of these and silhouettes of obviously armed guards behind him is the background on the screens for the buying/selling and hire/sacking of personnel.
* CoversAlwaysLie: The European cover art depicts insect-like aliens which do not appear in the actual game.
* CutscenePowerToTheMax: Inverted. The opening animation of ''UFO Defense'' features troops facing down Mutons with Personal Armor, a couple of the beginning rifles, and an Auto-Cannon. By the time you normally face Mutons, however, you'll be trampling them with Powered and Flying Suit-equipped soldiers packing any combination of Psi-Amps, Laser Rifles, Heavy Plasmas, or Blaster Launchers. Played straight in that you ''will'' get your ass kicked. Also, amusingly, in that you will never see a red-suited Muton in the game; the Muton Commander seen in the opening does not exist ingame. Mutons have no commanders.
* DroneOfDread: The battlescape soundtrack is a constant, low, pulsing drone.
* FacelessGoons: What your soldiers become when wearing power armor. A mod exists which makes it that they are holding their helmets in their hand in the inventory screen, like in ''TFTD''.
* FantasticDrug: Sort of. Some [=UFOs=] have weird rooms with walls that look like they're covered by shifting blue or orange clouds and have strange silver orbs with red cores. These rooms are revealed to be a form of alien entertainment that stimulate certain sensory regions of the brain. They are described as being similar to hallucinogenic drugs.
* [[FighterLaunchingSequence Drop Ship Launching Sequence]]: Seen in the Intro movie.
* FlyingSaucer
** One of the smaller alien craft in the first game take this form, as does the first X-COM-built craft.
** The Cyberdisc is essentially a miniaturized flying saucer with a powerful plasma cannon and self-destruct mechanism.
** The hovertanks appear to be based off the Cyberdisc design. As in, they just look like repainted Cyberdiscs with a tank cannon mounted on top.
** Essentially every UFO is a variation of a Flying Saucer.
* FormFittingWardrobe: Mutons wear green skin-tight body suits.
* GottaKillThemAll: Missions don't end until every martian is on the floor. Annoying when you've stunned one inadvertently: they'll get up and go for a wander after a while, leaving you to wonder why the mission's not ended after clearing out the UFO.
* MasterOfNone: The Lightning. It can intercept and carry troops, but is a worse fighter than the Firestorm and a worse troop bus than the Skyranger. Contrast the Avenger, a true JackOfAllStats.
* MiniMecha: The Sectopods, the heavily armored bipedal chicken-legged robot that serves as the terrorist unit for Ethereals.
* NextSundayAD: The original game was released in 1993 and set in 1999.
* NothingIsScarier: The combination of Night-time Missions and the Battlescape music is one of the perfect recipes for fear.
* {{Novelization}}: Has had at least two: [[http://en.wikipedia.org/wiki/X-COM_:_UFO_Defense_-_A_Novel One]] by Creator/DianeDuane and one by Russian sci-fi author [[http://en.wikipedia.org/wiki/Vladimir_Vasilyev_%28writer%29 Vladimir Vasilyev]].
* PowerPincers: The Chrysallids have them, used to hold their victims for zombification.
* PromotedFanboy: Otto Zander, the main character behind the ''X-COM Let's Play'' trilogy was made into an X-COM hero unit in the remake.
* PyrrhicVictory: [[spoiler:Retroactively added in the ''Apocalypse'' manual, which stated that soon after the victory on Cydonia, the world goes into a political and economical chaos while X-COM is all but disgracefully disbanded.]]
* RareRandomDrop: Technically, every alien UFO that you assault runs on a supply of Elerium, with each power source containing a stack of 50, which means that you ''should'', in theory, get all kinds of Elerium from all the [=UFOs=] you're shooting down. But then RealityEnsues: you are firing explosive weapons at a ship that then crashes, meaning that nine times out of ten, the power source, and its associated Elerium, will be destroyed long before you touch down. Even if they aren't, a stray bullet from the weakest weapon in the game can destroy the power source in battle, taking the Elerium with it. Landed [=UFOs=] are therefore highly sought, and should be carefully attacked, in order to maximize your Elerium theft.
* ReptilesAreAbhorrent: The Snakemen.
* RoboTeching: The blaster launcher is a missile launcher that sends it payload to a series of player defined waypoints, making any absurd trajectory possible. However, since the missile can't hit its waypoints with perfect accuracy, the most effective way to use it is to set a waypoint twenty feet above an enemy's head, then another waypoint directly on the enemy, causing the missile to arc over the target, then slam down, ensuring it will explode even if it misses.
* [[BlackCloak Saffron Cloak]]: The Ethereals.
* ShoutOut: The leader of the aliens is a huge Alien Brain that controls their HiveMind, not so different from ''Franchise/{{Metroid}}'s'' Mother Brain.
* ShutUpHannibal: ''UFO Defense'''s ending.
* SinisterGeometry: Cyberdiscs, the featureless tank unit of the Sectoids. While not as devastating as Sectopods, they fire 3 shots per turn and always show up in packs. Making matters worse, its slim shape makes the Cyberdisc difficult to hit from a lateral angle.
* SinisterSilhouettes: The bodyguards in the Buy[=/=]Sell, Hire[=/=]Sack screen.
* SlasherSmile
** The Chryssalids have one ''permanently''. [[DemonicSpiders Fitting]], really.
** Snakemen have a big toothy grin as well, but then again, they don't have any visible lips.
* SlippySlideyIceWorld: Arctic and Antarctic missions, thankfully without the slippy slidey part.
* SquishyWizard: Ethereals' bodies have atrophied so much that their self-sustaining functions have to be governed by their immense PsychicPowers. Which does not prevent them from having the best armour values and second best hit point totals among the non-terror unit aliens. [[FridgeBrilliance Their huge cloaks present a considerably larger profile then their actual bodies... hitting the cloak is not necessarily hitting the Ethereal.]] [[OrganDodge Plus, there isn't much to damage in their bodies.]] However, their brains still need an ample supply of blood, and if they lose too much blood because of serious injuries, they die.
* TrailersAlwaysLie: There are four details that make the Intro very misleading.
-->1. You will never see that cool-looking Dropship in the game.\\
2. You will never see that that Red Muton commander in the game.\\
3. You don't get Personal Armor right away.\\
4. Rifles and Auto-Cannons are hopelessly useless against Mutons.
* TheVeryDefinitelyFinalDungeon: The aliens' Martian base located in the Cydonia region.
* WeCanRuleTogether: [[spoiler:The brain offers this in the ending cutscene, but you shoot it.]]
* ZombieApocalypse: Any Snakeman terror mission can turn into this if not properly managed, due to [[DemonicSpiders those damned Chryssalids]]. Every civilian or trooper is a potential zombie, and zombies are the larval stage of new Chryssalids.
[[/folder]]

[[folder:Terror from the Deep]]
* AchillesHeel
** Tentaculats: Unlike the ''UFO Defense'' counterpart which can damage tanks, Tentaculats can't damage your SWS, and will still stupidly try to attack them anyway to no avail. So always bring one if you are expecting them.
** Lobstermen: While even the alien's own Sonic Cannons can barely scratch them, they are surprisingly vulnerable to both melee attack and thermal weapons -- if you haven't got the [[ThisIsADrill Vibro Blade]] line of research opened up yet, make sure you pack some Thermal Tazers and[=/=]or Thermal Shok Bombs.
** Triscenes: They can take some Sonic Cannon punishment, but its non-existent underside armor means a single cheap magna-blast grenade thrown under it will most often kill it.
* TheAestheticsOfTechnology: Aliens have a UsedFuture look.
* [[HumanPopsicle Alien Popsicle]]: The aliens use cryogenic stasis chambers to remain dormant for thousands of years. You can also sell these chambers as VendorTrash.
* AnimalMotifs: The smaller [=USOs=] have a manta-like design.
* AndIMustScream[=/=]TorturedMonster: The Bio-Drones.
* ApocalypseHow -- Continental: Failing to stop the aliens means their city-weapon rises from the deeps and kills pretty much everything. [[spoiler: When you win, the aliens still have the last laugh, as the destruction of T'leth results in a near-Planetary apocalypse.]]
* ArmlessBiped: Like the Reapers, the Triscenes don't have arms. Unlike the Reapers, they make up for it by carrying tow Heavy Sonic Cannons on their bodies.
* ArmorPiercingAttack : While there is an "Armour Piercing" damage type, they aren't good at penetrating armor. This job instead applies to [[ThisIsADrill Vibro Blades, Thermic Lances, and Heavy Thermic Lances]]. While you ''can'' kill the absurdly heavily armored Lobstermen without them (they take a ''maximum'' of 50% damage from ''most'' sources), once you realize that they take ''200%'' damage from those weapons, you'll be carrying them with you ''everywhere''.
* ArtificialStupidity: Are any of the aliens in a given mission equipped with Sonic Pulsers? They'll also be carrying either a Vibro Blade, Thermic Lance, or Heavy Thermic Lance -- and yet they never use them, even if they've run out of Sonic Pulsers[[note]]This is due to a bug: the aliens can't functionally switch weapons, and their AI routines don't include melee attacks unless they have a built-in melee function (Lobstermen and Calcinites, for example)[[/note]].
* AquaticMook: With the exception of the few surface-only Terror units, all Aliens are aquatic.
* AwesomeButImpractical: Sonic Cannon deals huge damage, but has extremely low rate of fire and a very small clip.
* BagOfSpilling: X-COM was disbanded after the First Alien War thanks to politics of funding nations, reduced to the underwater Elerium salvage team financed by a tycoon, until the arrival of aliens prompts the nations to resume funding.
** In addition, all the research from the First Alien War is functionally useless: alien alloys react poorly to seawater, meaning they can't be used for armor or weapons (invalidating plasma weaponry), and lasers are impractical underwater due to blooming and other issues (invalidating laser weaponry). Meaning you're fielding operatives with basic projectile weaponry and zero combat armor.
* BlobMonster: The Calcinites, although they were contained in a humanoid diving suit.
* BoardingParty: The Ship Terror Missions.
* BrainInAJar: The Bio-Drones.
* BrokeYourArmPunchingOutCthulhu: [[spoiler:The end of ''Terror from the Deep'' results in T'leth's destruction spreading chemicals all over the world's oceans and thus snapping a few links off the food chain. Oops]].
* CloningBlues: Just about all of the aliens are cloned.
* DerelictGraveyard: Small ones when recovering [=USOs=].
* DieselPunk: The aliens show elements of this.
* DividedStatesOfAmerica: People's Republic of Alaska.
* EenieMeenieMinyMoai: They show up on island terror missions in ''TFTD''. Must be some kind of fad in 2040.
* EldritchAbomination: The BigBad is the sleeping one from T'Leth, and he waits.
* [[EverythingsBetterWithDinosaurs Everything's Harder with Heavily Armored Cyborg Dinosaurs with Mounted Sonic Cannons]].
* FishPeople
** The Gillmen.
** The Lobstermen.
* FlyingSaucer
** What do you get when you take the cyberdisc, miniaturize it, waterproof it, replace the CPU with a still conscious human brain and replace the plasma cannons with a concentrated sound wave generated by [[AndIMustScream said brain's reactions to the inherent agony of the process]]? The answer is: The Bio-Drones.
** There's also the Dreadnought, TFTD's answer to UFO's Battleship, which almost looks exactly like its UFO counterpart, in that it looks like a giant Flying Saucer.
* GameBreakingBug
** ''Never'' research the Tasoth Commander[[note]]Fixed in the CD version, though[[/note]].
** If an explosive object is detonated by a stray shot from a Deep One, the aliens will freeze up and their turn never ends.
* GameplayAndStorySegregation: Molecular Control supposedly works through mind control implants, but aliens can control soldiers with no implants just fine.
* [[GiantEnemyCrab Giant Enemy Lobsters]]: The much feared Lobstermen. Until you acquire flying suits and mind control. As they are invulnerable to basic pistols, they become the best target practices. Or, when you get [[ThisIsADrill Vibro Blades.]]
* GlassCannon
** The Tasoths are a varient of this. All Tasoths have potent M.C. abilities, but on lower difficulties, aquanauts with high enough M.C. stats can easily use mind control on them.
** Deep Ones possess an attack that can kill an aquanaut in one hit, regardless of armor, but are fairly easy to kill.
* HarpoonGun: Your starting "standard rifle". And it sucks hard.
* HotSubOnSubAction: Your subs versus [=USOs=].
* HumanoidAbomination: The Deep Ones.
* AnIcePerson: Hallucinoids are prehistoric jellyfish that were modified to use a powerful chemical freezer. They attack by using a melee attack that literally freezes targets to death. They are supposed to have a similar ranged attack, but almost never use it due to a bug.
* IfItSwimsItFlies: [=USOs=] (Unidentified Submersible Objects) and your flying subs that intercept them. Both can fly over land, with a handwave stating that the engines are convertible to work in the air without issues, but weapons can only be fired underwater due to their design (a torpedo is not the same thing as a missile, for example).
* ImplacableMan
** The Lobstermen. [[{{Determinator}} They. Will. Not. DIE.]] Let's put it this way... unless you drop them with the heaviest melee weapons, chances are, they're [[OnlyAFleshWound actually unconscious, not dead]][[note]]Lobstermen take 20% damage from most weapons, with the exception of Gauss (30%) and Sonic (50%). Thermal weapons inflict full damage, but are relatively impractical due to melee range, or due to being a single-shot weapon. In addition, lobstermen have the highest armor value in the game, almost 20% higher than the best armor you can get for your aquanauts (which is the second best armor in the game), meaning that even if you DO hit the lobsterman, there's a good chance it's not going to do anything because your shot didn't get through the armor. Now imagine facing a full Dreadnought of these guys, which can be in excess of ''twenty-five'' lobstermen! And just in case you were reassured by the fact that they take 200% damage from drills, they have a natural melee attack that's as strong as a Thermic Lance. And they still have a gun in their hand. Enjoy![[/note]].
** The Triscenes are Implacable ''[[EverythingsBetterWithDinosaurs Dinosaurs]]''.
* KillItWithIce: Subverted. Freezing weapons, including Thermal Tasers and Thermal-Shok Bombs, are the equivalent of Stun Weapons from ''Enemy Unknown''. Instead of killing targets, they harmlessly freeze them, allowing you to capture aliens.
* LethalLavaLand: Sort of, one of the ''TFTD'' combat mission terrains consists of underwater mini-volcanoes leaking cooled lava. They have no effect on your soldiers and enemies, but they do provide illumination in night missions.
* LizardFolk: The Psychic Tasoth.
* LostColony: T'Leth is a massive colonizing ship that crashed 65 million years ago.
* MakeMeWannaShout
** The Aliens' Sonic Weapons, TFTD's equivalent of UFO's Plasma Weapons.
** Bonus points for the Bio-Drone, whose sonic beam is based on the original vocal cords of the brain that pilots it, meaning that it literally screams its enemies to death.
* MasterOfNone: The Gillmen, though they become JackOfAllStats at higher difficulties.
* MultiArmedAndDangerous: The Lobstermen, mostly cosmetical in regards of equipment, but not for their close combat ability.
* NewNeoCity: Neo-Japan
* NoOntologicalInertia: Killing the BigBad and destroying T'Leth makes all the remaining Zbrite inert, only good enough in large numbers, which is how they managed to send an Avenger to Mars for E-115 prospecting.
* NoWaterproofingInTheFuture: Weapons and other technology developed (or reverse-engineered) during the decades of fighting in ''UFO Defense'' are completely useless underwater, so, in ''Terror from the Deep'', you must restart the researches from scratch. Consider it a {{Justified|Trope}} BagOfSpilling... that is, unless you're a clever enough hacker to exploit the similarities of the ''UFO Defense'' and ''[=TFTD=]'' engines and [[OldSaveBonus carry over goodies whose quantities were stored in the same data addresses]]. Partially justified by the fact that the alien gear needs Elerium, and the rest of the stuff is lasers. Still doesn't explain why they don't keep a few crates of lasers around for land missions, though.
* PeopleJars: Alien Autopsy reports feature the Alien remains in a liquid tank.
* PhlebotinumKilledTheDinosaurs: The Colony ship T'leth crashing on Earth is what killed the dinosaurs.
* PowerPincers: The Lobstermen have them, naturally, and they hurt like hell.
* PunctuationShaker: T'leth.
* PyrrhicVictory: [[spoiler:Retroactively added in the ''Apocalypse'' manual, which states that the Destruction of T'Leth, in addition of killing your elite soldiers, releases the deadly chemicals that instantly kills everything in and around the Gulf of Mexico and turns the rest of the Earth into a toxic wasteland.]]
* RareRandomDrop: Averted after the Elerium fiasco of the first game: the destruction of a USO engine does not necessary destroy the associated Zrbite, making it much more common (so much so that, in the early game, you can actually sell it if you're desperately in need of money, since you won't need it for a few months anyway).
* ReactorBoss: The second part of alien colonies missions, and to lesser extent Artifact Sites, are too large to accomplish by killing every single alien, so it is more practical to find the synonium device that powers the base, destroy it while optionally capturing one of its high ranked guards, and get out.
* [[RecycledInSpace Recycled Underwater]]
* ReptilesAreAbhorrent: The Tasoth.
* RunningGag: A small one amongst veteran ''Terror'' players is that bases which see regular combat against the Lobstermen often requisition suspiciously large amounts of butter.
* SealedEvilInACan: The BigBad is one, not suprising since he is an expy of Cthulhu.
* SequelDifficultySpike: Early versions of the first game had a bug that enforced low difficulty mode. Due to player complaints, the sequel had the challenge rating cranked up -- sure the bug was gone, but even "easy" mode was a fair challenge without SaveScumming.
* ShipLevel: The Cargo[=/=]Cruise Ship Terror Mission. They consist of two parts, above deck and below deck, like Cydonia. Plus numerous rooms, narrow corridors and lots of hiding places and you got a recipe for disaster.
* ShockAndAwe: Researching a Deep One reveals that their "acid spit" attack is actually a highly dangerous electrical discharge. It is the only attack in the game that armor is completely unable to guard against.
* ShoutOut: ''TFTD'' is practically made of shout outs, if not direct ripoffs.
** The Calcinites in bear a laughable resemblance to the title antagonist of the B-movie ''Robot Monster'' due to them impersonating old-school divers.
** The Tasoth race are pretty much Lovecraft's Deep Ones (even though there's an entirely different race in the game actually ''called'' "Deep Ones"), especially since their original description (which was replaced in the final version of the game) had them being converted humans (much like the aforementioned actual Deep Ones of the final game).
** The Gill Men are extremely similar visually and thematically to the Gill Man of ''Film/CreatureFromTheBlackLagoon'', as well as being very reminiscent of the Sea Devils and Silurians of ''Series/DoctorWho''.
** The Great Dreamer, leader of the aliens, who sleeps most of the game away in the sunken spaceship/city called T'leth is, when you finally see him, an expy of Cthulhu, that giant monster/god dude who's slept away most of history in the sunken city called R'lyeh.
** The Tentaculats are, visually, a copy of ''D&D'' Grells, but a lot of that sort of thing went on in the early '90s.
* SpaceFillingEmpire: The Nations in 2040, Including:
** MiddleEasternCoalition: Arabic Bloc.
** UnitedEurope: Euro-Syndicate, with the exception of Northern Europe which formed it's own united state (Scandinavia) and Icelandic Union.
* TechnologyMarchesOn: CRT monitors in 2040 and much more.
* ThisIsADrill: The game features a series of power drills that are the most effective weapons against Lobstermen. Combine with [[MindControlDevice Molecular Control Disruptors]] to conserve ammo, or [[FreezeRay Thermal Shok Launchers[=/=]Thermal Tazers]] if you need to take them alive.
* UltraTerrestrials: The Gill Men are a lifeform native to Earth. Research implies that they are a human ancestor that, instead of staying on land, ended up in the water. There's also a lot of evidence of genetic manipulation on the alien's part, so they're a human off-shoot in name only at this point.
* UnderTheSea: Half of ''TFTD'' is this.
* UnderwaterBase: X-COM's base of operations. Also, one of the rarer terrain type in the USO recovery missions is a small series of underwater modules.
* UnderwaterRuins: Of several varieties. You can find sunken islands, sunken pirate ships (with gold! That you can't have), and even airplane carcasses.
* UnWinnable: The state of game in which the Tasoth Commander has been researched. Thankfully, patches and mods keep this (and other deadly bugs within the TechTree) from happening.
* UsedFuture: The Aliens seems to give this vibe.
* TheVeryDefinitelyFinalDungeon: The Colony Ship of T'leth.
* {{Vibroweapon}}: The drills.
* WaterIsAir: The ''Terror from the Deep'' was directly adapted from the original with no changes, so the characters are able to do ridiculous things like ''throwing grenades underwater.'' They also are unable to float or swim ([[WalkDontSwim instead just tromping around on the ocean bottom]]) until you research the equivalent of the flying suit[[note]]which is justified by the fact that they are in extremely heavy diving suits, so that the ocean current doesn't pull them away from the battlefield[[/note]]. On the Geoscape, there are times where your fighter craft[=/=]troop transport cannot engage[=/=]deliver soldiers due to the (downed[=/=]landed) USO being "too deep", due to water pressure: even the most basic USO is capable of surviving depths that would crush a human-made submarine. Once you have the Leviathan, USO interceptions will never be called off due to extreme depth.
* XtremeKoolLetterz: The Sonic-Blasta Rifle and Thermal-Shok Bomb Launcher.
[[/folder]]

[[folder:Apocalypse]]
* AbnormalAmmo
** Brainsuckers for the... Brainsucker Launcher.
** Also, the Entropy Launcher: A bioweapon firing homing missiles which release a compound that will dissolve through armor. It will also dissolve ''you'', should you be unarmored. As an added bonus, they'll make any explosives you're carrying... well, ''explode''.
* ActionBomb: The Poppers.
* TheAestheticsOfTechnology: The Aliens and their technology are organically ugly. The Mega-Primus city regulations mandates that everything should look retro-futuristic, which is most noticeable on flying cars.
* [[spoiler:AfterTheEnd: Thanks to the events of ''Terror from the Deep'', Earth is effectively a wasteland.]]
* AntiAir: Various weapon modules for the ground vehicles.
* AnotherDimension: The Alien Dimension.
* ApocalypseHow: Planetary, [[spoiler:finishing off the job from ''TFTD'']].
* ArmlessBiped: The Poppers. The larger weapon-platform type aliens don't really have proper arms either.
* AttackOfThe50FootWhatever: The Overspawn.
-->'''[[LetsPlay Director Zander]]:''' Oh please tell me that's not a fifty-foot monster.
** On a lesser scale, the Megaspawn and Psimorphs - the former being an organic weapon platform about 2-3 'floors' tall with built-in disruptor beam cannon and missile launcher. The latter is a powerful psionic entity a good 10-12 feet tall and packing SpareBodyParts.
* BeePeople: The Aliens.
* BlobMonster: Micronoid Aggregates. [[spoiler:They're really how the Brainsuckers take over their victims]].
* CorruptCorporateExecutive: Marsec, Solmine and Transtellar had a record of suppressing colonists rights.
* CorporateWarfare: The less scrupulous corporations wage secret wars against other corporations, and technically X-COM is itself a corporation in 2080s.
* CripplingOverspecialization: The Hybrids. They have great psychic powers and slightly greater reflexes than human soldiers, but are inferior in all other ways. Even worse, they improve their physical stats slowly, and you must assign them to a gym if you want to train their low strength -- i.e. they will either be unable to carry much, or they won't be able to train their psychic powers in the psy-gym. Finally, they need a special device to use their psychic powers, which means one of their hands will always be full -- i.e. they're usually restricted to using pistols, or suffering speed penalties.
* {{Cult}}: The Cult of Sirius, who believe that the Aliens are the saviors of humanity. They're the only faction in the game that will start off hating you. And, even if you spend (way) too much money to get their opinion to neutral, they'll never actually like you (the costs are prohibitive) and their opinion will nosedive as soon as you engage any alien forces. They're also (generally) hated by every other faction in Megaprimus. What this means...is that they are ''perfect'' raiding material for you to get additional money. Kill 'em all, let their god sort them out, and steal their stuff.
* DecapitatedArmy: Averted; you have to methodically and systematically destroy the entire alien infrastructure to win. Resistance wanes after the destruction of the Command Center, then resurges to reach its peak when you commence the final attack on the dimension gate generators themselves, with the surviving alien forces fighting like rabid, cornered animals.
* DeflectorShields: Disruptor Shields, for both individuals and vehicles.
* TheDevTeamThinksOfEverything: Selling a large amount of alien weaponry will result in it showing up in the hands of the various factions in the city, which can be either a very good thing or a very bad thing depending on your relationship with those factions.
* DoWellButNotPerfect: If you're very, very good, the Senate will decide that you're very cost-efficent and will reduce your weekly funding.
** Alien tech/type ramp-up is also dependent on scoring; do too well early on, and you're at risk of biting off more than you can chew.
* EnemyMine: Factions have a matrix of relationships, and an attack on given faction will cause those who are more hostile to the target to support the attack. If attacking aliens causes relationships to decrease with other corporations, then they like the aliens a bit more than they like X-COM.
* FantasticRacism: Sectoid Hybrids and Androids are at best treated as second-class citizens.
* FictionalPoliticalParty: Major parties of Mega-Primus Senate are NotSoDifferent rivals [[http://www.ufopaedia.org/index.php?title=Extropians Extropians]] and [[http://www.ufopaedia.org/index.php?title=Technocrats Technocrats]].
* FlyingCar: Relatively common vehicle type in ''Apocalypse''.
* FunWithAcronyms: The fact that a robots' rights group in ''Apocalypse'' would call itself the "Sentient Engine Liberation Front" clearly indicates that they deserve more credit than they're given.
* GunsAkimbo: The Real Time combat mode allowed this -- oddly, turn-based did not; carrying two guns penalized accuracy and only let you fire one at a time. Whilst troopers suffer (sometimes considerable) accuracy penalties for dual-wielding certain large weapons, it's quite feasible to use two autocannons at once if one so chooses. With a [[GameMod bit of tweaking]] for fully automatic fire and large magazines, you really have to be careful with that Explosive and Incendiary ammo.
* HeelFaceTurn: The Sectoids -- maybe. Played straight in the case of Sectoid Hybrids, who will serve you as loyally as any human soldier.
* HeroInsurance: Averted; you pay for the collateral damage.
* HotBlade: The Power Sword, a powerful blade weapon that is enhanced by a Elerium-powered plasma sheath.
* InfinityMinusOneSword: The androids are strong, tough, fast, very hard to tire out, almost impossible to make panic, and ''completely and utterly immune'' to two potentially devastating forms of attack; Brainsuckers, and Mind Control. Early and late-game, they make very dependable soldiers- but the price for this dependability is ''glacial'' stat-growth (so slow that most players will insist they have no stat growth at all) and inability to learn out of combat using combat facilities. Human soldiers will eventually come to outclass androids, but only after months of training and (potentially lethal) battlefield experience.
* InvisibilityCloak: The Personal Cloaking Field, which pops up very late in the game. It's not perfect (attacking from cloak causes it to shimmer and partially reveal the user) but it's pretty damn useful.
* IsometricProjection: In addition to traditional Isometric Battlescape, the Cityscape also uses this perspective.
* JustBeforeTheEnd: The aliens invade from a doomed, volcanic planet in another dimension where most other life has been scorched away by the local star's supernova.
* MegaCity: Mega-Primus itself, obviously.
* MegaCorp: About ten major ones in ''Apocalypse'', among a few others it'd probably be a good idea to defend against alien infiltration.
* MobWar: Rare, but happens sometimes.
* TheNudifier: The Entropy Gun. It's a homing bio missile which, on hitting, starts to dissolve all your armour and weapons.
* OrganicTechnology: Everything the Aliens use.
* OurWormholesAreDifferent: In the form of vaguely pyramid-like portals.
* PhlebotinumBomb: The Alien Gas grenades and missiles, hurt only aliens while doing nothing to humans.
* PointDefenseless: Laser Defense Arrays. The Plasma version is better.
* PuppeteerParasite: The Micronoids.
* RaygunGothic: The general art-style.
* RealTimeWithPause: One of two Battlescape modes. Each mode has its own pros and cons.
* ScrewTheRulesIHaveMoney: You can do this. No, ''really''. Even if ''every'' corporation in the city turns against you (either by subversion by the aliens or by hating you for any number of reasons) and the Senate ''ceases funding and threatens to shut you down'', you can keep fighting for survival so long as your bank account remains in the positive... and as long as you have a functional base and a steady source of income to sustain your private army, you can go ''renegade''. Want to show the Senate what you think of their threats? ''Go level half the city.'' [[EvilLaugh Mwhahahahahahaha!]]\\\
Sources of said income include: Raiding enemy corporations. Manufacturing and selling alien equipment. Selling captured alien equipment. Acting as a pusher for alien techno-drugs. It is strongly recommended that you cease employing conventional vehicles as soon as feasibly possible, as alien-derived craft do not require maintenance fees or fuel. Doing so can considerably reduce your maintenance bills.\\\
You cannot make an enemy of Transtellar, however, as they control all public transit. Civilians like your scientists and engineers require public transports to move from base to base, or to bring new hires to your labs. They cannot use your own vehicles for this, which means you can flip off the police, vaporise the assets of the Megacorps and violently depose the government but God help you if you annoy the [[AlmightyJanitor Taxi drivers]].\\\
Another reason for not making enemies out of everyone is that you'll be tormented with frequent base invasions if you annoy someone too much. This invariably results in the death of a few of your unarmed and unarmoured scientists, as well as being extremely irritating. What this boils down to is: Keep manufacturing equipment with the best profitability margins with your engineers for cash flow; keep raiding companies you don't like for equipment, while making your troops dualwield Devastator cannons and sweeping fire across their maps in real-time combat to make them so poor they cannot afford to raid you... and keep bribing Transtellar to keep their opinion of you maxed out. You can theoretically level the entire city except for Transtellar... including the government and the police.
* ShoutOut: Marsec and the M4000 autogun are a refrence to the early Gollop brothers game, Laser Squad.
* SpareBodyParts: Psimorphs are explained to be so resilient because they have as many as ''twelve'' copies of every major organ.
* TheSyndicate: Three of them.
* TakeOverTheCity: The Aliens' goal is take over Mega-Primus as a first step to taking over the world. Your mission is to invade the alien dimension and to destroy ''their'' city, one building at the time.
* TeleportersAndTransporters: Personal teleporters in ''Apocalypse''. Aliens appear to 'beam down' to buildings on the Cityscape with some form of white tube effect; not to be mistaken for the ominous blue [[spoiler:Micronoid Rain]].
* TubeTravel: The main form of pedestrian travel in Mega-Primus.
* UrbanSegregation: Outside of Mega-Primus there are few old, degraded buildings that is all that is left of old Toronto. Hybrids and Androids are forced to live here, while the gangs operate from there.
* UrbanWarfare: Most of the game takes place in one big city, so this is inevitable.
* UterineReplicator: Making babies the old-fashioned way is unpopular by the time of 2084, where they are now grown in Procreation Parks.
* TheVeryDefinitelyFinalDungeon: The alien dimension is an excessively long version of this trope.
* VirtualPaperDoll: You can combine different armor parts, mostly for Marsec Flying Armor torso part, which is the only part required to be able to fly.
* WhatMeasureIsANonHuman: Asked by S.E.L.F. and the Mutant Alliance as they fill X-COM's needs for robotic and [[HalfHumanHybrid alien-hybrid]] soldiers, respectively.
* WeBuyAnything: Strangely not averted. However, apparently [[AdamSmithHatesYourGuts Adam Smith Hates X-COM's Guts]], since market prices for player-manufactured items in ''Apocalypse'' can only go down.
* WeSellEverything: Subverted, as it required you to maintain good relations with a variety of {{Mega Corp}}s to obtain troops, aircraft, weapons, safety from police interference, etc.
* WeirdTradeUnion: The various organizations supply their respective goods and personnel to X-COM. Fail to protect them from aliens or just piss them off, and they will stop making business with you.
[[/folder]]

[[folder:Interceptor]]
* ApocalypseHow: If you fail, the alien superweapon utterly obliterates Earth, leaving a smoldering husk behind. You get to turn the tables on them, though, [[spoiler:using the Nova Bomb to cause a supernova in their pocket dimension solar system]]. Technically, you can do it ''as much as you want'', which is a little frightening.
* DoomsdayDevice: The Aliens' Project Doomsday and our own Nova Bombs.
* TheFinalFrontier / SettlingTheFrontier: One of our side jobs is to protect the developing outposts that will later mine Elerium.
* GoneHorriblyRight: The [[http://www.youtube.com/watch?v=z655q7pClO8 cutscene]] when you complete research on the Nova Bomb, which of course includes a test. The pilot who reports that the test was successful doesn't sound too happy about it. [[SlasherSmile But the player's avatar is.]]
* {{Interquel}}: Set between ''TFTD'' and ''Apocalypse''.
* MyBrainIsBig: The Psilords are apparently all brain.
* SpaceFighter: The main part of the game is piloting one.
* SpacePirates
* SpaceStation
* StarKilling: The Nova bombs.
* TheVeryDefinitelyFinalDungeon: The Star system housing the Doomsday project, hidden [[spoiler: on the other end of the event horizon of a ''black hole'', not only necessitating your fighter piggybacking on another carrier to get in and out, but justifying the use of the Nova Bomb.]]
[[/folder]]

[[folder:Enforcer]]
* AllDesertsHaveCacti
* AndYourRewardIsClothes: Each difficulty level you beat Enforcer at in single play unlocks a few new skins for the title robot.
* AttackDrone: The Enforcer itself, and it can get one for itself.
* GaidenGame: Set during the First Alien War.
* HoldTheLine: One early mission. It will make you want to scream and cry at the same time.
* MythologyGag: ''Enforcer'' features a mishmash of resources from two fellow ''X-COM'' titles scrapped during its development, ''Genesis'' and ''Alliance''.
[[/folder]]

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->[[WrittenSoundEffect *pew* *blam*]] "Aughgghggurglegurgle..." [[HellIsThatNoise *slither* *slither* *pit* *pat* *tap* *click-click*]] '''*BOOM*''' "Aiiieeegurgle gurgle..."
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