Achievements

Left 4 Dead includes achievements for Steamworks and Xbox LIVE. There are 51 achievements in all for the PC version of Left 4 Dead (not including the 7 from Survival Mode, or the 10 from Crash Course), but only 50 for the Xbox 360 version (not including the 10 from Crash Course); the Outbreak achievement is not in the Xbox 360 version of the game. Then there are currently 50 known achievements for Left 4 Dead 2. In the upcoming downloadable campaign, The Passing, there will be new achievements added to Left 4 Dead 2.

Note that none of these achievements can be achieved with cheats turned on.

Cheats can no longer be used to get achievements, as of Crash Course update. If cheats are ever turned on in the programs session,(sv_cheats = 1) achievement progress and unlock will immediately be locked until restart of the program. Note that achievements cannot be reset, so if you decide to cheat then there is no way to get the achievement later without cheating.

Survival Mode
None of the Survival Achievements are present in the Xbox 360 version of the game since it is free DLC.

Mutations
will take you six weeks to get this achievement

Xbox 360-Exclusive
with the passing you can unlock 2 new avatar awards the Gnome, and a Left 4 Dead 2 hat

The Gnome

play 6 mutaions

Left 4 Dead 2 Hat

play any level of the passing

Unknown achievements
The following are up and coming achievements for The Passing but aren't known whether they are achieved through Versus, campaign, or the new game play modes.

as you play through the passing you will find boxes full of items they are what you need to open for the achievement