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