Full list of all 69 Halo: Reach achievements worth 1,700 gamerscore. It takes around 40-50 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 49 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 20 achievements worth 700 Gamerscore.
Reached the rank of Captain in the UNSC.
Advanced a Commendation to a Silver state.
Finished the 9th mission on Legendary with the Scorpion intact.
Completed a Weekly Challenge.
Won a game of Invasion in the 1st phase.
As an Elite, killed 5 Spartan players in Matchmaking.
Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
Hijacked a Banshee during the Reach Campaign.
Killed 2 vehicles at once with the Target Locator during the 3rd mission.
Completed the Campaign on Legendary difficulty.
Completed the Campaign on Heroic difficulty.
Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
Completed all of the Daily Challenges in a given day.
Cleared the 2nd mission without setting foot in a drivable vehicle.
Killed 7 Moa during the 2nd mission of the Campaign.
Performed an Assassination against an Elite to survive a fall that would've been fatal.
Completed the 10th mission on Normal or harder.
Completed the Campaign on Normal difficulty.
Completed the 9th mission on Normal or harder.
Completed the 8th mission on Normal or harder.
Killed 3 of the anti-aircraft batteries during the 8th mission.
Completed the 7th mission on Normal or harder.
Completed the 6th mission on Normal or harder.
Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
Completed a Firefight set on Legendary without dying.
Earned a Killionaire medal in Firefight.
Scored 1,000,000 points in a Firefight game.
Scored 200,000 points in a Firefight game.
Scored 50,000 points in a Firefight game.
Completed the 5th mission on Normal or harder.
Killed the Elite Zealot before he escaped during the 5th mission.
Completed the 4th mission on Normal or harder.
Completed the 3rd mission on Normal or harder.
Completed the 2nd mission on Normal or harder.
Killed 10 enemies in a Firefight or Campaign session with the DMR.
Traded weapons with an AI ally in Campaign.
Earned a Killing Spree in multiplayer Matchmaking.
Earned a First Strike Medal in a Matchmaking game.
Recommended a file to someone.
Used the File Browser to upload a file to your File Share.
Purchased an item from the Armory.
Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
Reached the rank of Corporal in the UNSC.
Scored 15,000 points in Score Attack Firefight Matchmaking.
Used a Health Pack to replenish life after taking body damage.
Performed an Assassination on an enemy.
Complete each Campaign Mission on Legendary - alone.
Purchase an item from the Armory that requires the rank of Lt. Colonel.
Noble 1: Earned a kill in a matchmade Rocket Race game.
Noble 1: Destroyed a vehicle using Armor Lock in a matchmade game.
Noble 1: Killed a player at long range with the DMR in a matchmade Slayer game.
Noble 1: Earned a Double Kill from the Grave in multiplayer Matchmaking.
Noble 1: Earned a Double Kill with the shotgun in multiplayer Matchmaking.
Noble 1: Collect all of the flags in a matchmade Stockpile game.
Noble 1: Blow up the research facility in a matchmade Invasion game on Breakpoint.
Defiant: Captured two flags from an enemy capture zone in a matchmade Stockpile game.
Defiant: Killed 3 players with the Oddball in a matchmade Oddball game.
Defiant: Returned two flags in a matchmade CTF game.
Defiant: Prevent the attacking team from touching the flag in a matchmade 1-Flag CTF game.
Defiant: Assassinate a player that just exited Armor-Lock in a matchmade game.
Defiant: Be the last surviving uninfected player in a matchmade Infection game.
Anniversary: Infected 3 players in a single round in a matchmade Infection game.
Anniversary: Killed the flag carrier with a Plasma Grenade in a matchmade game.
Anniversary: Performed a Beat Down on an enemy while using the Jetpack during a matchmade game.
Anniversary: Killed an enemy with their own grenade that was stuck to you in a matchmade game.
Anniversary: Captured 10 skulls at once in a matchmade Headhunter game.
Anniversary: Earned 3 headshots in a row in a matchmade game.
Anniversary: Run over 5 people with a vehicle in a matchmade game.