Left 4 Dead is a multiplayer co-operative survival horror first-person shooter game from Valve Corporation. The game has been built using Valve's Source engine.

Left 4 Dead pits four human playable or AI controlled survivors of an apocalyptic pandemic against hordes of aggressive "Infected". 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.