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.
Beat the game on Impossible difficulty.
Win a multiplayer match.
Beat the game in Ironman mode on Classic or Impossible difficulty.
Beat the game on Classic difficulty.
Mind Control an Ethereal. Single player only.
Complete a Very Hard abduction mission in five turns or less on Classic or Impossible difficulty.
Kill 50 aliens with explosive weapons.
Kill 500 aliens.
Clear a UFO crash site with one soldier on Classic or Impossible difficulty.
Beat the game on any difficulty.
Make contact with the Ethereal hive mind.
Shoot down 40 UFOs.
In a single game, complete every Research Project.
In a single game, complete every Foundry project.
Staff the Research Labs with 80 scientists.
Staff the Engineering Department with 80 engineers
In a single game, shoot down one of each alien craft.
Win a mission with an all-female squad. Single player only.
Build the Gollop Chamber.
In a single game, complete every Autopsy.
Successfully assault an Overseer UFO.
Kill an alien while flying. Single player only.
Build a suit of powered armor.
Earn the "Excellent" rating in every performance category on a terror mission.
Cure poison on five soldiers in a single mission. Single player only.
Build the Hyperwave Relay.
Kill 150 aliens.
Build a Firestorm.
Find a soldier with the Gift.
Build a S.H.I.V.
Gain access to the lowest level in your base.
Advance one of your soldiers to Colonel rank.
Recover the Hyperwave Beacon.
Get satellite coverage over every country on one continent.
Uncover the alien base's location.
Acquire 1000 credits in one month.
Stun an Outsider.
Fulfill a Council request.
Build a Workshop.
Capture a live alien.
Build a Laboratory.
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.
Have a soldier survive every mission in a full game.
Win the game from each of the 5 starting locations.
Take your new ally into the final challenge.
Complete Gangplank.
Complete Confounding Light.
Complete Confounding Light with more than 3 turns left on the timer.
Complete Friends in Low Places.