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

Realistic Difficulty (50 points): Complete single player story on "Realistic" difficulty.
Co-op Realistic Difficulty (50 points): Complete the co-op story on "Realistic" difficulty.
Quality Time (20 points): Invite a friend to join and participate in a co-op story or game mode session.
Hunter (10 points): Complete any 1 map in "Hunter" game mode in co-op.
Last Stand (10 points): Complete any 1 map in "Last Stand" game mode in co-op.
Hunter Completionist (20 points): Complete all maps in "Hunter" game mode on rookie or normal difficulty.
Hunter Master (50 points): Complete all maps in "Hunter" game mode on realistic difficulty.
Last Stand Completionist (20 points): Complete all maps in "Last Stand" game mode on rookie or normal difficulty.
Last Stand Master (50 points): Complete all maps in "Last Stand" game mode on realistic difficulty.
Face-Off Completionist (20 points): Complete all maps in "Face-Off" game mode using any connection type.
Face Off (10 points): Win one match in "Face-Off" game mode on any difficulty.
Preparation Master (30 points): Complete all prepare & execute challenges.
Stealth Master (30 points): Complete all vanish challenges.
Best Of The Best (30 points): Complete all Splinter Cell challenges.
Well-Rounded (50 points): Complete all challenges.
Weapon Upgraded (10 points): Purchase all 3 upgrades for any 1 weapon.
Gadget Upgraded (10 points): Purchase all 3 upgrades for any 1 gadget.
Weapons Expert (20 points): Purchase all 3 upgrades for all weapons.
Gadgets Expert (20 points): Purchase both upgrades for all gadgets.
Weapons Collector (20 points): Unlock all weapons in the weapon vault.
Variety (10 points): Purchase any 1 uniform.
Accessorizing (10 points): Purchase any 1 accessory for any 1 uniform.
Ready For Anything (20 points): Purchase all 9 accessories for all uniforms.
Fashionable (20 points): Purchase all 6 texture variants for all uniforms.
Perfect Hunter (20 points): Complete any map in Hunter without ever having been detected on realistic difficulty.
Last Man Standing (50 points): In Last Stand, survive all enemy waves of any map in one session without failing on any difficulty.
Revelations (20 points): Discover Anna Grimsdottir's dark secret.

Additionally, there are 18 secret achievements:

Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (20 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.
Secret achievement (10 points): Unknown.