Left 4 Dead is a cooperative first-person shooter video game. It was developed by Turtle Rock Studios, which was purchased by Valve Corporation during development. The game uses Valve's proprietary Source engine, and is available for Windows and the Xbox 360. Development on the game was completed on November 13, 2008 and was released digitally on November 17, 2008 and at retail on November 18, 2008 in the United States and on November 21, 2008 in Europe.

Set during the aftermath of an apocalyptic pandemic, the game pits its four protagonists—dubbed the "Survivors"—against hordes of the infected. There are four game modes: a single-player mode in which allied characters are controlled by AI; a four-player, co-op campaign mode; an eight-player online versus mode; and a four-player survival mode. In all modes, an artificial intelligence (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 by the industry media upon its release, with praise given for its replay value, focus on cooperative play, and movie-like experience. Several criticisms were aimed at limited level selection and the lack of a narrative. The game has won several publication awards, as well as distinctions from the Academy of Interactive Arts & Sciences and British Academy of Film and Television Arts. As was done with Team Fortress 2, Valve plans on supplementing the game with free downloadable content. The first of these, called the "Survival Pack", was released on April 21, 2009, with the second, in the form of a new DLC Campaign entitled "Crash Course," released for both the PC and Xbox 360 on September 29, 2009. The popularity of the game led to the development of a sequel, Left 4 Dead 2, which was released November 17, 2009. Left 4 Dead will be brought to Mac OS X along with other Valve games in mid-2010, allowing for cross-platform support between PC and Mac versions.