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.
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 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.
Kill 50 aliens with explosive weapons.
Kill 150 aliens.
Kill 500 aliens.
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.
In a single game, complete every Research Project.
Kill an alien while flying. Single player only.
In a single game, complete every Autopsy.
Find a soldier with the Gift.
Capture a live alien.
Stun an Outsider.
Uncover the alien base's location.
Build the Hyperwave Relay.
Build the Gollop Chamber.
Build a Firestorm.
Launch a Satellite.
Get satellite coverage over every country on one continent.
In a single game, shoot down one of each alien craft.
Build a suit of powered armor.
Cure poison on five soldiers in a single mission. Single player only.
Complete a mission without losing a soldier.
Advance one of your soldiers to Colonel rank.
Acquire 1000 credits in one month.
Staff the Research Labs with 80 scientists.
Staff the Engineering Department with 80 engineers
In a single game, complete every Foundry project.
Build a S.H.I.V.
Build a Laboratory.
Build a Workshop.
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 a Research Project.
Build a base facility.
Build an item.
Gain access to the lowest level in your base.
Fulfill a Council request.
Successfully assault an Overseer UFO.
Recover the Hyperwave Beacon.
Shoot down a UFO.
Complete the tutorial mission.
Make contact with the Ethereal hive mind.
Win a mission with an all-female squad. Single player only.
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.