The brainchild of Julian Gollop and other assorted Microprose personnel, ''UFO: Enemy Unknown'' was 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'', since there was a naming rights conflict with an American-made DOS shooter called ''UFO''. By either name, ''X-COM'' puts the player in command of an e''X''traterrestrial ''COM''bat unit charged with protecting Earth from an alien threat, managing resources and researching captured technology in the process. The hybrid of RealTimeStrategy (improving X-COM's overall condition and catching [=UFOs=] as they land - or crashing them yourself) and TurnBasedStrategy (exploring crash sites, stopping terror attacks, and defending and assaulting bases) quickly won the hearts of the gaming public.

Almost 15 years after its initial release, ''UFO Defense'' still attracts players and tops lists of the Best PC Games of All Time. [[http://pc.ign.com/articles/772/772285p1.html A 2007 assessment by IGN]] has it edging out fellow Prodigal Son of Microprose ''Sid Meier's {{Civilization}} IV'' for the Number 1 slot.

Not to say the ''X-COM'' legacy is a solo act, however. While Gollop's team set to work on ''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'' hit the shelves in 1997, to mixed reviews due to its ArtShift into pseudo-3D futuristic graphics and the clunkiness of a newly-introduced real-time option for playing missions. The last days of Microprose (and its acquisition by Hasbro Interactive) saw ''X-COM'' trying to get back on its feet with two [[GenreShift Genre Shifted]] offerings: ''X-COM: Interceptor'' (1998) kept the base management elements while swapping out the strategy missions for space-bound [[SimulationGame Flight Simulator]] action, while ''X-COM: Enforcer'' (2001) ditched the strategy part outright to make a FirstPersonShooter running parallel to the timeline of ''UFO Defense''. Sadly, neither had the mystique of their ancestors, and are often [[DisContinuity shunted away from canon]] due to the UnexpectedGenreChange (and in the case of ''Enforcer'', being awful).

While the possibility of a future ''X-COM'' game continues an infinitely long march towards zero due to the license being passed through a variety of incapable hands throughout the 2000s, the earlier games attract a variety of player-made mods and remake attempts in numerous stages of completion. Various [[SpiritualSuccessor Spiritual Successors]] also exist, such as ''[[UFOAfterBlank UFO: Aftermath]]'' and its sequels ''Aftershock'' and ''Afterlight'' (unrelated to the game ''UFO'' listed above), the Game Boy Advance sleeper ''Rebelstar: Tactical Command'', and ''Laser Squad: Nemesis'' (in and of itself a sequel to ''X-COM'''s own predecessor ''Laser Squad''). All have attracted moderate attention from ''X-COM'' fans, largely for either the similarity in gameplay (the ''UFOAfterBlank'' series) or the connections to Gollop and other former ''X-COM'' staff (''Rebelstar'' and ''Laser Squad'').

Whether due to the entire series being re-released on {{Steam}} or the runaway success of [=GuavaMoment=]'s [=~Let's Play~=] of ''Apocalypse'', ''X-COM'' has experienced a resurgence among retro gamers, especially those eager to [[FanficRecs/{{X-COM}} chronicle their campaigns]].

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!!Not Enough Trope Units!

* AbnormalAmmo: Brainsuckers for the... Brainsucker Launcher.
**Also, the Entropy Launcher: A bioweapon firing homing missiles which release a compound that will dissolve through armor.
* AbusivePrecursors: The Alien Brain in UFO Defense claims that the aliens created humanity.
* AfterTheEnd: Thanks to the events of Terror From The Deep, Earth is effectively a wastleland.
* AlienBlood: Green and Yellow seems to be most common ones.
* AlienInvasion: Duh.
* AliensAndMonsters: Mostly aliens, but their Terror Units are often engineered (genetically or otherwise) to either capitalize on their owner's strengths (Chrysalids 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).
** But what of the Mutons' Terror Units? Silacoids and Celatids are more GoldfishPoopGang material.
*** Superior to the Mutons in one aspect: [[DoctorWho They are better at dying.]]
* AliensStealCattle: Several missions involve "Harvester" [=UFOs=] sent to meet the aliens' carnivorous needs. [[IncrediblyLamePun They are, of course, equipped with ]] CowTools.
* 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.
* 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.)
* AndYourRewardIsClothes: Each difficulty level you beat Enforcer at in single play unlocks a few new skins for the titular robot.
* AnotherDimension: The Alien Dimension in Apocalypse.
* AnticlimaxBoss: The Alien Brain at the tail end of ''UFO Defense''. See the page for examples of just how ''pitiful'' the final fight can get.
* AnyoneCanDie: And anyone WILL die until you've got solid armor research going. Or even afterward.
* ApocalypseHow: Varies from Class 3 to Class X.
* ArmorIsUseless: A soldier with maximum health and the best armour can still be killed in one shot if the damage roll is high enough.
** To clarify, X-COM soldiers take 0 to 200% of the listed damage from firearms. 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.
*** On the other hand, the better armors available for X-Com Agents render at least normal weak weapons useless against them. Of course, the only benefit of being immune to rifle bullets is that if your friend carrying a normal rifle gets mind-controlled, he can't even dent you by shooting you in the back.
*** In addition, the Power Suit also makes it possible for agents to use Autocannons loaded with explosive ammunition in close-quarters combat with little risk of being badly injured by the splash damage.
** Truly badass and lucky troopers can claim to survive point-blank Blaster Bomb detonations thanks to this.
** 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.
* ATeamFiring: Most recruits will hit [[http://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.
* AttackDrone: Cyberdiscs and Bio-Drones.
** Enforcer's protagonist. Recursively, he too can get an AttackDrone.
* AttackOfThe50FootWhatever: The Overspawn from Apocalypse.
---> [[LetsPlay Director Zander]]: Oh please tell me that's not a fifty-foot monster.
* 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 [[NerfArm Nerfed]] by the game mechanics.
* BadassNormal: 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.
* [[BlackCloak Saffron Cloak]]: The Ethereals.
* 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.
* BodyHorror: Chrysalids, Bio-Drones, Tentaculats, several of the ''Apocalypse'' aliens, and so forth.
* 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).
* BrainInAJar: The Bio-Drones.
* {{Brainwashed}}: The common state of victims of Ethereals, high-ranking Sectoids, and their successors. Often, they're [[BrainwashedAndCrazy also crazy]].
* BrokeYourArmPunchingOutCthulhu: The end of ''Terror From the Deep'' results in [[spoiler:T'leth's destruction spreading chemicals all over the world's oceans and thus snapping a few links off the food chain. Oops]].
* BulletholeDoor: Great for reducing the effects of DroneJam during Terror Missions. Busting through the walls of [=UFOs=], however, will take well-placed/lucky plasma holes (interior) or Blaster Launcher shots (exterior).
* 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 absolute 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.
* 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.
* CombatMedic: Anyone with the medkit, and boy, you gonna need them.
** EVERYONE should have a medkit. No exceptions.
* 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.
* CriticalEncumbranceFailure: Of the "carry items up to the soldier's Strength in weight, then take Time Unit penalties for going overboard" type.
* CriticalExistenceFailure: [[AvertedTrope Averted]] - Soldiers 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.
* {{Cult}}: The Cult of Sirius.
* 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.
** Amusingly played straight 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.
* DeathFromAbove: The Floaters. Players can also do this once they research a means of flying.
** Some Enforcer enemies will do this. In parcticular 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.
* DeflectorShields: Disruptor Shields in ''Apocalypse'', for both individuals and vehicles.
* DemonicSpiders: In the early game, candidates abound- Chryssalids (able to go from off-screen to melee attacking in one turn) and Ethereals (champions of psi-spamming) deserve special mention as frustrating and unfair enemies. Later in the game, this is reversed and a strong squad is virtually invincible, not in the least because ''you'' are the one making with the psi-spamming at that point.
** Chryssalids have as many as ''80'' or more Action Points/Time Units on the hardest difficulty, and as little as 60 on the lowest. To move one tile takes 4, or 6 if diagonal. Thus, they can blitz your troops from alleyways and corners. On top of that, if your suprised agents kill the Zombies created by a Chryssalid with reaction fire, out pops a ''new'' Chryssalid.... with FULL TIME UNITS and ready to make more Zombies out of your men. Chain reactions of frightened soldiers killing zombies with Reaction Fire before getting zombified by the freshly hatched Chyrssalids are the number one cause of {{Total Party Kill}}s when battling against Snakemen.
** There's also a fair share of these in TFTD. Tentaculats take it to a whole new level with their ability to [[NightmareFuel ''fly'' when underwater]]. Combine this with the fact they ''love'' to hide in little nooks and crannies (which exist in alien bases for this sole purpose, it seems), one Tentaculat can ''decimate'' an ill-prepared attack force single-handedly.
*** And Bio-Drones. Don't you just love enemies that upon being shot, immediately spin around and return fire with 100% accuracy?
* DerelictGraveyard: Small ones when recovering USOs in TFTD.
* DestroyableItems: Repeat after me. No grenades or rockets in the alien engine room.
* DisasterDominoes: What happens when your units are low on morale.
* DoomsdayDevice: Project Doomsday from Interceptor.
* DroneJam: Civilians are NOT your friends during Terror Missions. They move about at random, block your movement from place to place in the process, provide excellent breeding opportunities for Chrysalids outside of your visual range, and turn hostile when you mind-control them away from enemy fire (thankfully, as they're unarmed, it's a token hostility). If you're not particularly concerned with their well-being, [[TheUriahGambit then what's another few human casualties alongside your squaddies?]]. Otherwise, check the page for nonlethal civilian control methods of varying degrees of effectiveness and hilarity.
* DropShip: Ranger and Triton from UFO and TFTD respectivly.
* DummiedOut: One of the many things left unused is the "Alien Reproduction" item and research line in ''UFO Defense'' - strange considering the resultant HalfHumanHybrids 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.
* 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.
* DynamicDifficulty: Regardless of the difficulty your campaign starts at, most of the games will see fit to ramp up alien activity to correspond with consistent positive performance.
** The bug in UFO Defense caused the difficulty to scale back to beginner. Because of that, gamers complained that the game was (relativly) easy, which made the developers of TFTD to increase the difficulty across the board. And here where the fun part comes in: because of the the unchanged engine, the difficulty resetting bug is still there, only apparently it goes to superhuman instead of beginner.
*** If you don't eventually go for the BigBad in TFTD, alien bases will start to proliferate faster than you can keep up with them.
* 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).
** Blaster Bombs could certainly count, as well as Cyberdiscs, particularly when chain reactions are caused. But as we already know, they're MadeOfExplodium.
** Repeated explosions will start to dig a hole in the ground. In ''UFO Defense'' this hole is purely a visual artifact as eventually concrete or a road will be destroyed exposing dirt...at the same level. In TFTD, most things take place on ground anyways but it still happens in port terror missions. In Apocalypse, as the LetsPlay demonstrates, it's possible to accidentally end up digging an enemy that can withstand multiple missile hits a foxhole.
* EasyLogistics: Averted so very hard. While ammunition for conventional weapons can be bought as long as you have 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.
** Moreso when [=XComUtil=]'s "Improved Laser Weapons" fix is implemented. Sure, the Heavy Laser finally gets Auto Shot capability, but must it come at the cost of using Elerium for Laser construction AND not being able to make Plasma weapons (even after taking into consideration that the aliens drop Heavy Plasmas like candy)?
* EenieMeenieMinyMoai: They show up on island terror missions in TFTD. Must be some kind of fad in 2040.
* ElaborateUndergroundBase: A necessity due to X-COM's covert nature, often leading to AllYourBaseAreBelongToUs should the aliens stumble upon it (hopefully "them" by the time an Alien Retaliation fleet comes calling).
** Aliens get these, too. Which you have to break into to kidnap high-ranking officers for interrogation to complete the game.
** Enforcer does this in a few levels.
* EldritchAbomination: The BigBad from the TFTD is the sleeping one from T'Leth, and he waits.
* EliteMooks: Alien Squad Leaders in general. Later-Game Aliens (Mutons, Tasoth, Lobstermen) also may count.
** Enforcer bosses, which are usually giant versions of other mooks like Reapers or Chryssalids, with special attacks.
* 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]].
* 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.
* FaceFullOfAlienWingWong: Chryssalids, ''oh God, the Chryssalids''... Tentaculats and Brainsuckers fill their niche in ''Terror From The Deep'' and ''Apocalypse'', respectively.
**Brainsuckers represent an ''absolutely straight'' case of FaceFullOfAlienWingWong, no less, attaching onto your head and [[NightmareFuel emptying their innards down your throat]].
** Brainsuckers cross it with PeoplePuppets. Fortunately, unlike their earlier brethern, Brainsuckers die when taking over their victims, so it's not quite as bad.
* FakeDifficulty: ''[=TFTD=]'' was 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. [[LampshadeHanging Lampshaded]] regularly in the LetsPlay.
* FlyingSaucers: 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.
** The hovertanks appear to be based off the cyberdisk design. As in, they just look like repainted cyberdisks with a tank cannon mounted on top.
** 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.
* FishPeople: The Gillmen.
* FrickinLaserBeams: The Laser weapons obviously.
* 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.
* GameBreaker: Arguably, human Psionic capability in UFO Defense. In TFTD it was nerfed. Then ''Apocalypse'' rolls around...
** Special mention must be made of the almighty Blaster Bomb, with a monstrously powerful explosion of an 11-tile radius and the abiliy to punch holes into heavily armored Battleship hull plating. If its amazing stopping power isn't enough to impress, the fact that it can be guided via waypoints to weave ''through'' the corridors of a ship (and even ''up'' or ''down'' elevators!) to kill off a specific enemy should be enough to make soldiers clamor for it when you load one in the Skyranger. Suffers from CoolButInefficient at times though.
* GameBreakingBugs: ''Never'' research the Tasoth Commander.
* GeoEffects: On the Strategic scale, where you land determines what kind of terrain it will be in the battlefield. 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.
* [[GiantEnemyCrab Giant Enemy Lobsters]]: The much feared Lobstermen aliens in ''Terror of the Deep''.
* GiantMook: Many terrorist aliens. The Megaspawn from Apocalypse. Enforcer bosses.
* GoodBadBugs: Most every release and patch of ''UFO Defense'' has introduced or fixed a couple of these. Whether they are to be exploited or patched depends on how NintendoHard you want your X-COM experience to be.
** Sadly, [[GameBreakingBug their evil twins]] are just as commonplace in ''Terror From The Deep''.
** A fun exploit involves hitting certain sides of the UFO with explosives; due to imperfect wall structure, explosions can "leak" through certain parts of the UFO hull. Roast the aliens in their (UFO) shell...
** This troper is a fan of standing on a bale of hay so you can see and shoot through the ceiling at the aliens on the floor above. Especially given that they're usually camping at the top of the stairs to ambush you when you come up.
* GlobalCurrency: Everything bought and sold in the main games is apparently done so in U.S. dollars.
* 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 transport could consume 40 of those slots just giving each soldier a gun and its ammunition. And that's without bringing extra ammo for reloading.
* GunsAkimbo: Apocalypse's 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.
* HighOctaneNightmareFuel: "Ooh, what's this? [[DemonicSpiders Chrysalids]]? [[SchmuckBait I wonder how this'll turn out]]!"
* HiveMind: The aliens in ''UFO Defense'' take orders from one, another in ''Terror From The Deep'' tries to play CosmicHorror while it's at it, and the Biomass in the ''UFO'' games is a weapon designed to [[GeniusLoci turn a planet into one]].
* 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.
** One early Enforcer mission. [[ThatOneLevel It will make you want to scream and cry at the same time]].
* {{Hovertank}}: With your choice of Fusion Bomb launcher or Plasma cannon. Arguably, the Cyberdisc can be considered one of these sans turret.
* HumanoidAliens
* IfItSwimsItFlies: USOs (Unidentifed Submerged Objects) from the second game and your flying subs that intercept them. Both can fly over land, but your subs cannot fire unless underwater.
** It would've been nice to bring in local air forces and then recover the wreck on land with leftover laser weapons.
* 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).
* ImperialStormtrooperMarksmanshipAcademy: Best way to describe the accuracy of any Rookie. They can literally open a door, find themselves [[OhCrap toe-to-toe with an alien]], fire repeatedly at it, and still miss. Bonus points if their Shooting Accuracy is low. More bonus points if attempting to Auto Fire. Jackpot if they're suffering from Fatal Wounds in one or both arms. (The aliens' snap shots tend to be considerably more accurate ...)
** Ocasionally subverted by the odd rookie trooper who has uncannily high accuracy and can out-shoot some of your crack troopers.
* ImplacableMan: The Lobstermen. [[DeTerminator They. Will. Not. DIE.]]
* 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-egnineer 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.
* JustBeforeTheEnd: The aliens in ''Apocalypse'' invade from a doomed, volcanic planet in another dimension where most other life has been scorched away by the local star's supernova.
* KillItWithFire: Burning the zombies (with incendiary ammunition) will also kill the chryssalid without it popping out.
* 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.
* LivingShip: The Apocalypse UFOs.
* LizardFolk: The Psychic Tasoth from TFTD.
* LostColony: T'Leth is a massive colonizing ship that crashed billions years ago.
* MegaCorp: About ten major ones in ''Apocalypse'', among a few others it'd probably be a good idea to defend against alien infiltration.
* 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.
* MoneyForNothing: UFO parts and corpses sell quite well and X-Com itself can self-finance through arms manufacturing.
* 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).
* MyBrainIsBig: The Sectoids and Aquatoids obviously.
** Interceptor's Psilords are apparently all brain.
* NextSundayAD: The original game was released in 1993 and set in 1999.
* NightmareFuel: Chrysalids again. But they're just the tip of the iceberg once you factor in Bio-Drones in ''Terror From The Deep'', what with the majority of them being [[HumanResources still-conscious human brains]] [[BrainInAJar strapped onto a submersible weapons platform]] and [[BrainwashedAndCrazy implanted with alien control modules]].
** Don't forget the Tentaculats, which are outright described as being Lovecraftian. They're giant, floating, eyeless brains. With tentacles. ''And beaks''.
*** So they're just like the [[http://www.wayaadisi.com/chainmail/graphics/deathknell/grell.jpg Grell]] from [[DungeonsAndDragons D&D]]?
**** [[http://www.ufopaedia.org/index.php?title=Image:Tentaculat.PNG Worse.]]
* NintendoHard: At least two cases:
** Psionic enemies before you get psionic troops (Mind control hell)
** Any fight against a battleship when the doors on the bottom are propped open (by a dead body) or destroyed (Blaster bombs will hit you)
** Treat these as: "Sacrifice 1/4th your troops to let the others escape, or sacrifice 3/4ths of your troops for a chance of success". (So I play [[MagicTheGathering M:TG]])
*** Terror From The Deep in general, where because of the (then unknown) bug in UFO Defense locking the difficuly on Beginner prompted the fans to complain about it being (relatively) easy, which prompted the developers to make TFTD's Beginner setting as hard as UFO's Superhuman, not knowing that the same bug transferred over due to same engine, which now locks on Superhuman (UFO's superhuman +4) instead. Welcome to Hell.
* 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 [[JustifiedTrope Justified]] 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.
* 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 two types of scientist: Quantum physics and Biologists.
* OneHitPointWonder: Your units actually have a life bar but, until they gain lots of experience or get some armor, they might as well have one hit point.
* 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.
* 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''. The ''UFO'' games invert this, with bio-weaponry becoming more and more scarce throughout the sequels.
* 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]]).
* ProperlyParanoid: Anyone who takes ''great'' care while handling Terror Missions. If you aren't covering all the angles, you're just TemptingFate.
* PsychicPowers: Of the telepathic kind, making victims panic (or go berserk) to controling them.
* PsychicStatic: Cyborgs in Apocalypse can not be controlled at all.
** Presumably, neither can the protagonist of Enforcer.
* PunctuationShaker: T'leth from TFTD.
* ThePuppetMasters: The Micronoids from Apocalypse.
* 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.
* RandomNumberGod: See ArmorIsUseless trope above.
* [[RecycledInSpace Recycled in Underwater]]: Terror from the Deep.
* RaygunGothic: The general art-style of Apocalypse.
* RealTimeStrategy: During the Geoscape.
* RedShirt: The default state of every X-COM [[NewMeat recruit]]. [[MauveShirt Turning them Mauve]] is a LuckBasedMission in itself.
** Not quite a challenge since you can abuse [[SaveScumming save scumming]]. Once you have those psi devices, you can make the aliens throw away their guns and turn them into target practice for your new recruits.
* RedShirtArmy: What you start the game with.
* {{Roboteching}}: Blaster Launchers and their diabolical waypoint-based targeting system.
* SaveScumming: a common strategy, unless you think this is cheating or dishonorable, it is possible to win the game with 0 casualties.
* SequelDifficultySpike (To a game bug related to difficulty)
* ScaryDogmaticAliens: Of the Conqueror type.
* ScrewTheRulesIHaveMoney: You can do this in Apocalypse. 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 FightingForSurvival 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.
* SealedEvilInACan: The BigBad of TFTD is one, not suprising since he a expy of Cthulhu.
* 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).
* {{Sequelitis}}: After the original, the overall quality of each successive installment of the series was lower than the previous, regardless of their genre.
* [[CapcomSequelStagnation Sequel Stagnation]]: Averted. After ''Terror From The Deep'', which was basically [[MissionPackSequel an underwater rehash of the first episode]], new elements and even [[GenreShift Genre Shifts]] were introduced - unfortunately, they didn't result in good games.
** YourMileageMayVary. Apocalypse was fairly good, introducing elements like collapsable terrain pseudo-physics and had the real-time combat mode, but ''Interceptor'' and... [[strike:That Which Shall Not Be Named]] ''[[strike:[[{{Narm}} Enforther]]]]'' ''Enforcer'' are looked down upon. The latter so hard that it is considered completely [[DisContinuity non-canon]].
*** The mileage for Enforcer varies quite a bit; viewed purely as a run-and-gun arcade shooter, in the SmashTV line of things, it can be hugely entertaining. Viewed purely as an X-COM game, however, it's sort of like meeting the Antichrist.
* ShinyLookingSpaceships: UFOs of the first game. X-Com fighters in Interceptor.
* ShoutOut: The Calcinites in ''Terror From The Deep'' bear a laughable resemblance to the titular antagonist of the B-movie ''Robot Monster'' due to them impersonating old-school divers. ''Enforcer'' also features a mishmash of resources from two fellow ''X-COM'' titles scrapped during its development, ''Genesis'' and ''Alliance''.
* ShutUpHannibal: UFO Defense's ending.
* SlippySlideyIceWorld: Artic and Antarctic missions, thankfully without the slippy slidey part.
* 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.
* 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.
** Their huge cloaks present a considerably larger profile then their actual bodies...hitting the cloak is not necessarily hitting the Ethereal.
* StopHelpingMe: The scientist in Enforcer will leave you with a pathological need to kill him. Sadly, even at the beginning when you can actually ''see'' and ''shoot'' him, it has no effect.
* StrongFleshWeakSteel: Individual soldiers, in the late game, are ''far'' stronger than tanks, since soldiers improve their stats and tanks don't.
** 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.]]
** However, a hovertank/launcher can still fire fusion balls without ever needing to reload until they run out of ammo. Although, this is balanced out by having a maximum of 8 fusion balls per mission and doing less damage than blaster bombs. Also, tanks can't be stunned or get fatal wounds. Or Mind Controlled.
* 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.
* SuperSoldier: Mutons and Lobstermen. Your soldiers will become this if they lucky enough.
** Enforcer's protagonist, who racks up over a hundred dead aliens a mission, and sometimes as many as four hundred fifty.
* TankGoodness: HWPs are a refreshing alternative to the hopeless rookies in the early game, at least in ''Enemy Unknown''.
* TakeCover: Very important, given the computer's cheating tendencies and the power of alien weapons. Unfortunately, most forms of cover can be destroyed.
* 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...
* ThisIsADrill: Terror From The Deep features a series of power drills that are the most effective weapons against Lobstermen. Combine with Mind Control to conserve ammo.
* 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.
* TurnBasedStrategy: The Squad-Level type during the battlescape.
* UnderTheSea: Half of TFTD is this.
* UnderwaterBase: X-Com's base of operations in TFTD. Also, one of the rarer terrain type in the USO recovery missions is a small series of underwater modules.
* UnderwaterRuins
* {{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.
* UnWinnable: The state of any ''Terror From The Deep'' game in which the Tasoth Commander has been researched. Thankfully, patches and mods keep this (and other deadly bugs within the TechTree) from happening.
* 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.
* UsedFuture: The Aliens of TFTD seems to give this vibe.
* VideogameCaringPotential: Just TRY not to get attatched to your soldiers who got promoted to Sergeant or above rank. I dare you to.
* 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.)
** [[AndyWaltfeld This Troper]] markets his dead Ethereals as "Psycho Jerky." (Sectoid officers count as a mild flavor.)
** To be fair, 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 aliens' Mars base in UFO: Enemy Unknown, T'leth in X-Com: Terror from the Deep and heck, even the hidden star system in Interceptor.
** Bear in mind that that last one is 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.]]
* ViolationOfCommonSense: Someone explain to me again why buildings in ''Apocalypse'' have their "crumple points" on the ''freakin' roof?''
** So that when you accidentally level a corp's HQ, they have a reason to turn hostile and start invading your base. And then go "No, we love the aliens, so you can't appease us with money".
** Windows in Enforcer are good for about ten research points when destroyed. A dead Sectoid or Snakeman is worth ''one''.
* WeaksauceWeakness: Speaking of Sectopods, a misprint in their EncyclopediaExposita entry 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.
** Tentaculats in TFTD, 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 in TFTD while even the alien's own Sonic Cannons can barely scratch them, they are surprisingly vulnerable to melee attack, that includes tazers if you haven't got drills yet.
* WeHaveReserves: Rookies make good scouts.
* WeSellEverything: Played straight until ''Apocalypse'', which required you to maintain good relations with a variety of [[MegaCorp Mega Corps]] to obtain troops, aircraft, weapons, safety from police interference, etc.
** Strangely, WeBuyAnything is not similarly averted. However, apparently [[AdamSmithHatesYourGuts Adam Smith Hates X-COM's Guts]], since market prices for player-manufactured items in ''Apocalypse'' can only go down.
*** It should be noted that although WeBuyAnything is in effect, you should ''never sell your Entropy Guns.'' Seriously, don't do it. At that point in the game, some corporations will have been taken over by the aliens and you'll have effectively given the enemy access to their InfinityPlusOneSword.
* 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...
** With this lack of basic night-vision technology, you must fight at night on land, and underwater, and in buildings full of smoke a thermal viewer could see through easily. (But see below.)
* WhatMeasureIsANonHuman: Asked by S.E.L.F. and the Mutant Alliance in ''Apocalypse'' as they fill X-COM's needs for robotic and [[HalfHumanHybrids alien-hybrid]] soldiers, respectively.
* 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.
* Zeerust: Handheld night-vision equipment was still costly, encumbering and not very good in the early 90s.
* 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 casuality 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 their ufo doors lets you snipe them as they poke their heads out.
** One-robot Zerg Rush is probably the best way to play Enforcer, as it lets you get those research points.
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