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