Left 4 Dead is an upcoming multiplayer co-operative survival horror first-person shooter game by Valve Corporation. The game has been built using Valve's Source engine and will be available on Steam and the Xbox 360.

Left 4 Dead pits four human playable or AI controlled survivors of an apocalyptic pandemic against hordes of aggressive "Infected" in a cinematic backdrop in which the players "star" as each character in a fictional movie (eg. No Mercy.) The Survivors' goal is to help each other make it through various scenarios to safety. The 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 mutated Boss Infected with special abilities and cooperate to stop the Survivor players.