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