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