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