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