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 pack containing 5 achievements worth 100 Gamerscore.
In a single game, complete every Foundry project.
Beat the game on any difficulty.
Win a mission with an all-female squad. Single player only.
In a single game, shoot down one of each alien craft.
Win a multiplayer match.
Make contact with the Ethereal hive mind.
Kill 500 aliens.
Staff the Research Labs with 80 scientists.
Kill an alien while flying. Single player only.
Find a soldier with the Gift.
Build the Gollop Chamber.
Staff the Engineering Department with 80 engineers
In a single game, complete every Autopsy.
Successfully assault an Overseer UFO.
Acquire 1000 credits in one month.
Build the Hyperwave Relay.
Kill 50 aliens with explosive weapons.
Get satellite coverage over every country on one continent.
Build a Firestorm.
Build a S.H.I.V.
Gain access to the lowest level in your base.
Recover the Hyperwave Beacon.
Kill 150 aliens.
Uncover the alien base's location.
Build a suit of powered armor.
Stun an Outsider.
Build a Workshop.
Advance one of your soldiers to Colonel rank.
Earn the "Excellent" rating in every performance category on a terror mission.
Build a Laboratory.
Fulfill a Council request.
Capture a live alien.
Build a base facility.
Shoot down a UFO.
Launch a Satellite.
Complete a mission without losing a soldier.
Build an item.
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 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.
In a single game, complete every Research Project.
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.
Complete Gangplank.
Take your new ally into the final challenge.
Complete Confounding Light with more than 3 turns left on the timer.