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.
Shoot down 40 UFOs.
In a single game, complete every Research Project.
Have a soldier survive every mission in a full game.
Mind Control an Ethereal. Single player only.
In a single game, shoot down one of each alien craft.
Beat the game on any difficulty.
Make contact with the Ethereal hive mind.
Build the Gollop Chamber.
In a single game, complete every Foundry project.
Staff the Engineering Department with 80 engineers
In a single game, complete every Autopsy.
Successfully assault an Overseer UFO.
Find a soldier with the Gift.
Build the Hyperwave Relay.
Build a Firestorm.
Win a mission with an all-female squad. Single player only.
Kill an alien while flying. Single player only.
Recover the Hyperwave Beacon.
Staff the Research Labs with 80 scientists.
Kill 500 aliens.
Kill 50 aliens with explosive weapons.
Acquire 1000 credits in one month.
Gain access to the lowest level in your base.
Uncover the alien base's location.
Build a S.H.I.V.
Complete the tutorial mission.
Build a Laboratory.
Build a suit of powered armor.
Capture a live alien.
Kill 150 aliens.
Stun an Outsider.
Advance one of your soldiers to Colonel rank.
Build a Workshop.
Get satellite coverage over every country on one continent.
Earn the "Excellent" rating in every performance category on a terror mission.
Build a base facility.
Fulfill a Council request.
Shoot down a UFO.
Launch a Satellite.
Complete a Research Project.
Build an item.
Complete a mission without losing a soldier.
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.
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.