Left 4 Dead is a co-operative, survival horror, first-person shooter game by Turtle Rock Studios, who were purchased by the Valve Corporation part way into development. The game uses the Source game engine, and is available for Windows-based personal computers and the Xbox 360.

Left 4 Dead puts four human playable or AI-controlled Survivors of an apocalyptic pandemic against hordes of aggressive "Infected" (zombies). These Infected are controlled by an AI that dynamically balances difficulty and mood depending on the players' progress and situation. In an alternate game mode, human players can control up to four different monsters with special abilities and cooperate to stop the Survivor players.

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.A five-minute trailer was released on Halloween.A playable demo was made available on November 6 for pre-purchasers and on November 11 for the general public, and was closed on November 18, 2008. The demo contained the majority of the first two chapters in the "No Mercy" campaign, and was playable in both single player and multi-player.

Left 4 Dead was well received upon its release, with praise given for its replayability, focus on cooperative play, and movie-like experience. Receiving an aggregate score of 88% on Game Rankings and 89% on Metacritic. Similar to Team Fortress 2, Valve intends to support the game with downloadable content.