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