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