Full list of all 85 XCOM: Enemy Unknown achievements. It takes around 200 hours to unlock all of the achievements in the base game on Windows.
The base game contains 50 achievements, and there are 2 DLC packs containing 35 achievements.
Beat the game on Classic difficulty.
In a single game, shoot down one of each alien craft.
In a single game, complete every Research Project.
Kill an alien while flying. Single player only.
Clear a UFO crash site with one soldier on Classic or Impossible difficulty.
Complete a Very Hard abduction mission in five turns or less on Classic or Impossible difficulty.
Win a mission with an all-female squad. Single player only.
Beat the game on any difficulty.
Make contact with the Ethereal hive mind.
Kill 500 aliens.
Build a Workshop.
Staff the Engineering Department with 80 engineers.
Find a soldier with the Gift.
In a single game, complete every Foundry project.
Build the Gollop Chamber.
Staff the Research Labs with 80 scientists.
In a single game, complete every Autopsy.
Build a Firestorm.
Acquire 1000 credits in one month.
Gain access to the lowest level in your base.
Successfully assault an Overseer UFO.
Get satellite coverage over every country on one continent.
Build a S.H.I.V.
Build a suit of powered armor.
Build the Hyperwave Relay.
Kill 50 aliens with explosive weapons.
Advance one of your soldiers to Colonel rank.
Recover the Hyperwave Beacon.
Kill 150 aliens.
Capture a live alien.
Stun an Outsider.
Uncover the alien base's location.
Launch a Satellite.
Complete a mission without losing a soldier.
Build a Laboratory.
Complete a Research Project.
Build a base facility.
Build an item.
Fulfill a Council request.
Shoot down a UFO.
Complete the tutorial mission.
Beat the game in Ironman mode on Classic or Impossible Difficulty.
Win a multiplayer match.
Mind Control an Ethereal. Single player only.
Beat the game on Impossible difficulty.
Shoot down 40 UFOs.
Have a soldier survive every mission in a full game.
Earn the "Excellent" rating in every performance category on a terror mission.
Play a multiplayer match against someone with this achievement.
Win the game from each of the 5 starting locations.
Complete Confounding Light.
Complete Gangplank.
Complete Friends in Low Places.
Take your new ally into the final challenge.
Complete Confounding Light with more than 3 turns left on the timer.
Beat the game without buying a Squad Size upgrade (Classic+ difficulty)
Designate a highly decorated soldier as the Volunteer
Complete Furies
Field a squad where all members have at least two modifications (min. 4) and win the mission
Deploy three tactical subsystems on a single soldier in single player
Complete Deluge
Eliminate all alien waves
Get at least 4 kills with XCOM Base Security personnel
Field a squad consisting entirely of augmented soldiers and SHIVs (min. 4) and win the mission
Deal with the newest global threat
Make certain of the new threat's location
Get a Soldier to have 5 modifications in single player
Kill a Muton Berserker in melee combat in single player
Kill an elite enemy Sniper with one of your own snipers in single player
Kill an elite enemy Medic with explosive damage in single player
Cause an enemy to suicide
Jump two stories in one move in single player
Eliminate an enemy's shield and kill it on the same turn in single player
Use Collateral Damage to blow up a car in single player
Eradicate the infestation
Beat a new special mission without losing any assets
Have your special-duty soldier kill three enemies in the same mission
Augment a soldier in single player
Kill an unseen enemy detected by a specially modified soldier in single player
Proc Combat Rush on the entire squad (min. 4)
Field a fully enhanced squad and win the mission
Modify a soldier in single player
Stop a squad member from suffocating in single player
Field a squad with 4 augmented soldiers, each with a different base ability and win the mission
Kill an enemy as it is psionically attacking you in single player