Grand Master (150 gamerpoints): You completed 100 challenges.
Reach for the Stars (50 gamerpoints): You completed 50 challenges.
The Russian Hare (20 gamerpoints): You performed 47 headshots using a sniper rifle.
A Taste for the Game (20 gamerpoints): You completed 10 challenges.
Absolution (50 gamerpoints): You completed Hitman: Absolution™ as a professional.
Jack of All Trades (30 gamerpoints): You collected all 20 play styles.
It's All in the Wrist (50 gamerpoints): You successfully performed a lethal throw.
Information is Power (30 gamerpoints): You collected all evidence.
Thumbs Up (20 gamerpoints): You liked a contract.
Damage Control (20 gamerpoints): You contained a situation gone bad.
Set for Life (20 gamerpoints): You earned 1 million contracts dollars.
Self-improvement (20 gamerpoints): You bought an upgrade for a weapon.
First Contract (20 gamerpoints): You completed the Creating Contracts Tutorial.
Blood Money (20 gamerpoints): You completed the Contract Basics Tutorial.
True Potential (20 gamerpoints): You unlocked a technique.
Partners in Crime (20 gamerpoints): You played a contract made by a friend.
Competitive Spirit (20 gamerpoints): You created a contract competition.
Contender (20 gamerpoints): You participated in a contract competition.
One With the Shadows (20 gamerpoints): You escaped the attention of an enemy.
Silent Assassin (20 gamerpoints): You achieved Silent Assassin.
Inconspicuous (20 gamerpoints): You remained undetected throughout a checkpoint.
Whoops (20 gamerpoints): You made a kill look like an accident.
One of the Guys (20 gamerpoints): You blended in and fooled someone.
Rocksteady (20 gamerpoints): You executed a point shooting with at least 3 kills.
Under Wraps (20 gamerpoints): You hid a body.
Sandman (20 gamerpoints): You subdued a person.

There are also 20 secret achievements.