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