Left 4 Dead (L4D) is a cooperative first-person shooter video game. It was developed by Turtle Rock Studios, which was purchased by the Valve Corporation part-way into development. The game uses the Source engine, and is available for Windows and the Xbox 360. The game went gold on November 13, 2008, and was released on November 18, 2008, in the United States; and on November 21, 2008, in Europe to coincide with the tenth anniversary of the release of Half-Life.[2][3]

Set during the aftermath of an apocalyptic pandemic, the game pits its four protagonists?dubbed the "Survivors"?against hordes of the Infected, zombie-like aggressive mutants. There are three game modes: a single-player mode in which allied characters are controlled by AI; a four-player, co-op Campaign mode; and an eight-player Versus mode. In all modes, an AI, dubbed "The Director", controls level pacing and item spawns, in an attempt to create a dynamic experience and increase replay value.

Left 4 Dead was well received with an aggregate score of 88% on Game Rankings and 89% on Metacritic upon its release, with praise given for its replay value, focus on cooperative play, and movie-like experience. As was done with Team Fortress 2, Valve released new downloadable content for the game, on April 21, 2009.