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 on Impossible difficulty.
Beat the game in Ironman mode on Classic or Impossible difficulty.
Have a soldier survive every mission in a full game.
Beat the game on Classic difficulty.
Win a multiplayer match.
In a single game, complete every Research Project.
Win a mission with an all-female squad. Single player only.
Clear a UFO crash site with one soldier on Classic or Impossible difficulty.
Mind Control an Ethereal. Single player only.
Beat the game on any difficulty.
Kill 50 aliens with explosive weapons.
Shoot down 40 UFOs.
Complete a Very Hard abduction mission in five turns or less on Classic or Impossible difficulty.
Make contact with the Ethereal hive mind.
Cure poison on five soldiers in a single mission. Single player only.
Find a soldier with the Gift.
Build the Gollop Chamber.
In a single game, complete every Foundry project.
In a single game, complete every Autopsy.
Successfully assault an Overseer UFO.
In a single game, shoot down one of each alien craft.
Kill 500 aliens.
Earn the "Excellent" rating in every performance category on a terror mission.
Stun an Outsider.
Uncover the alien base's location.
Acquire 1000 credits in one month.
Staff the Engineering Department with 80 engineers
Build a Laboratory.
Gain access to the lowest level in your base.
Kill an alien while flying. Single player only.
Recover the Hyperwave Beacon.
Staff the Research Labs with 80 scientists.
Build the Hyperwave Relay.
Build a S.H.I.V.
Build a suit of powered armor.
Build a Firestorm.
Advance one of your soldiers to Colonel rank.
Get satellite coverage over every country on one continent.
Kill 150 aliens.
Build a Workshop.
Fulfill a Council request.
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.
Complete Confounding Light with more than 3 turns left on the timer.
Take your new ally into the final challenge.
Complete Gangplank.
Complete Confounding Light.
Complete Friends in Low Places.