Achievements

Left 4 Dead includes achievements for Steamworks and Xbox Live. There are 73 achievements in all for the PC version of Left 4 Dead, but only 65 for the Xbox 360 version; the Outbreak achievement is not in the Xbox 360 version of the game, and the Survival Pack on Xbox 360 adds no new achievements due to technically being only a patch, not a DLC. There are currently 101 achievements for the PC version of Left 4 Dead 2 (two of which are date-dependent) and 65 achievements for the Xbox 360 version.

Note: In Left 4 Dead, since the Crash Course update, achievements cannot be earned with cheats turned on, however, one way is to play on Versus mode with everyone helping each other to get achievements. Another easy way to obtain achievements is to create a server in order to exploit the console commands.
 * Survival Pack adds 7 new achievements to Left 4 Dead for the PC version only.
 * Crash Course adds 10 new achievements to Left 4 Dead.
 * The Passing adds 10 new achievements to Left 4 Dead 2.
 * The Sacrifice adds 5 new achievements to both Left 4 Dead and Left 4 Dead 2, and for the Xbox 360 and PC as well as Mac.
 * The Cold Stream update added 2 new achievements to Left 4 Dead 2 called Stream Crosser and Connecting Fights, although the latter achievement is actually keyed to the conversion of the Dead Air campaign in Left 4 Dead 2. As a reward for 60,000 players earning the Connecting Fights achievement, Valve released a beta version of the Blood Harvest campaign for Left 4 Dead 2.
 * A special holiday-themed achievement, called "Valve Gift Grab 2011 — L4D2", was added to the PC version of Left 4 Dead 2 for a limited time in late December 2011.
 * The achievement Good Guy Nick is achieved by surviving a campaign with players who obtained their copy of the game during a free weekend.
 * The Last Stand Community Update adds 30 new achievements to the PC version of Left 4 Dead 2.
 * A PC-exclusive achievement titled Gnome Alone was added to Left 4 Dead 2 in late November 2020.

Full Campaign Achievements
Note: for all the following achievements, it is recommended to play on Crash Course, as it is only 2 chapters long.

Survival Mode
None of the Survival Achievements are present in the Xbox 360 version of the game since it is an update and not DLC.

Miscellaneous
Achievements that don't fit anywhere else.