Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Adjudicate the Arbiter (30 points): Complete the Campaign on Heroic Difficulty.
Detour the Great Journey (50 points): Complete the Campaign on Legendary Difficulty.
Momma's Boy (5 points): Get a Gold Medal on any mission.
Crushed Colors (10 points): Improve your Score in any Campaign Mission.
Mr. Punctual (15 points): Finish all missions under Par Time on Heroic difficulty Own Worst Enemy (20 points): Get a Gold Medal with all Debuff Skulls Active.
Wall of Recognition (30 points): Get a Gold Medal on every Campaign Mission .
Epic Grinder (40 points): Obtain a Lifetime Campaign Score over 1,000,000.
Everything's Better with Bacon (5 points): Mission 1: Ram 50 Grunts with Warthogs.
Endless Fun (5 points): Mission 2: Destroy every Methane Tank.
Covenant "Hot Drop" (5 points): Mission 3: Kill at least 5 Covenant Units with the Bridge.
The Real Winner (5 points): Mission 4: Save Adam.
He's Got The Jack (5 points): Mission 5: Jack 6 Covenant Vehicles.
Rhino Hugger (5 points): Mission 6: Successfully protect every Rhino.
Micro Manager (5 points): Mission 7: Do not destroy any Power Node.
Ramblin' Man (5 points): Mission 8: Use Elephants to train 100 Infantry.
Sweet Naptime (5 points): Mission 9: Put every Colony in Hibernation Mode at the same time.
The Procrastinator (5 points): Mission 10: Disrupt all Tractor Beams.
Battened Down the Hatches (5 points): Mission 11: Save all the Airlocks.
Handy with Tools (5 points): Mission 12: Repair the Power Core in less than 4 minutes.
Beaming with Pride (5 points): Mission 13: Destroy 25 units with the Scarab.
Didn't Get To Second Base (5 points): Mission 14: Don't claim an extra base.
Thinkin' about My Doorbell (5 points): Mission 15: Open the Doors in order.
Backscratcher (10 points): Complete any Campaign Mission in Co-op Mode.
OMG BFF FTW (40 points): Complete entire Campaign in Co-op Mode.
Playin' the Field (15 points): Win a Skirmish Game with every Leader.
Gallivant around the Galaxy (25 points): Win a game on every Skirmish Map.
Empire Builder (5 points): Win a game in every Skirmish Game Mode.
Titan (15 points): Get 20,000 points in any Skirmish Game.
Big Al's Scooter (10 points): Win a Heroic Skirmish Game against the AI in under 10 minutes.
My Virtual Friends Love Me (10 points): Win a 3v3 Skirmish game with 2 AI Allies.
Walk-Off Winner (30 points): Use one of the 6 Major Leader Powers to destroy an enemy's last unit.
2 Bugs are Better Than 1 (10 points): Win a Skirmish Game with Dual Scarabs.
Penny Pincher (10 points): Get a winning High Score with 10 or less Squads against the Heroic AI.
N00b n0 M0r3 (10 points): Win a Matchmade Skirmish Game on Xbox LIVE.
So Lonely at the Top (20 points): Win and have the Highest Score in a Matchmade Skirmish Game on Xbox LIVE.
Basically Naive (10 points): Obtain the Recruit Rank on Xbox LIVE.
Officer on Deck (30 points): Obtain the Lieutenant Rank on Xbox LIVE.
Running the Show (50 points): Obtain the General Rank on Xbox LIVE.
Alas, Poor Andrew Thomas (5 points): Collect your first Skull.
Graverobber (30 points): Collect all Skulls.
Halo Academic (15 points): Unlock 20 Timeline Events.
Halo Historian (40 points): Unlock All Timeline Events.
Ready for the Sequel (75 points): 100% Completion.
24 Hours of Quality (20 points): Play Halo Wars for at least 24 Total Hours.

Additionally, there are five secret achievements, all of which are currently unknown:

Secret achievement (10 points): Unknown.
Secret achievement (40 points): Unknown.
Secret achievement (50 points): Unknown.
Secret achievement (70 points): Unknown.
Secret achievement (70 points): Unknown.