Full list of all 55 XCOM: Enemy Unknown achievements worth 1,100 gamerscore. It takes around 80-100 hours to unlock all of the achievements in the base game.
The base game contains 50 achievements worth 1,000 Gamerscore, and there is 1 DLC packs containing 5 achievements worth 100 Gamerscore.
Achievements they've won
Achievements they've not won
Beat the game on any difficulty.
Make contact with the Ethereal hive mind.
Mind Control an Ethereal. Single player only.
In a single game, complete every Research Project.
In a single game, shoot down one of each alien craft.
Build the Gollop Chamber.
Find a soldier with the Gift.
In a single game, complete every Autopsy.
Successfully assault an Overseer UFO.
Build the Hyperwave Relay.
In a single game, complete every Foundry project.
Recover the Hyperwave Beacon.
Kill an alien while flying. Single player only.
Kill 500 aliens.
Build a Firestorm.
Staff the Engineering Department with 80 engineers
Staff the Research Labs with 80 scientists.
Build a S.H.I.V.
Build a suit of powered armor.
Build a Laboratory.
Advance one of your soldiers to Colonel rank.
Build a Workshop.
Get satellite coverage over every country on one continent.
Acquire 1000 credits in one month.
Kill 50 aliens with explosive weapons.
Win a mission with an all-female squad. Single player only.
Kill 150 aliens.
Uncover the alien base's location.
Earn the "Excellent" rating in every performance category on a terror mission.
Gain access to the lowest level in your base.
Stun an Outsider.
Fulfill a Council request.
Capture a live alien.
Build a base facility.
Shoot down a UFO.
Launch a Satellite.
Build an item.
Complete a Research Project.
Complete a mission without losing a soldier.
Complete the tutorial mission.
Beat the game in Ironman mode on Classic or Impossible difficulty.
Win a multiplayer match.
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.
Shoot down 40 UFOs.
Have a soldier survive every mission in a full game.
Cure poison on five soldiers in a single mission. Single player only.
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 Friends in Low Places.
Complete Confounding Light.
Take your new ally into the final challenge.
Complete Confounding Light with more than 3 turns left on the timer.