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