Set in a modern day survival-horror universe, the co-operative gameplay of Left 4 Dead™ casts four "Survivors" in an epic struggle against hordes of swarming zombies and terrifying "Boss Infected" mutants.

The Survivor Co-op Game Mode is played out across four sprawling campaigns, set in urban and rural environments. The team of four Survivors may be comprised of one to four human players, allowing for single player and multiplayer co-op games.

The next-generation A.I. technology of all non-player Survivor and zombie characters is a pool of systems used to procedurally generate a unique experience by monitoring the players’ experiences and tailoring the pacing and difficulty of action for each game. This makes for a different experience each time the game is played.

In the Left 4 Dead Vs. Mode, play as a Survivor or as one of four types of "Boss Infected." Each of the "Boss Infected" possesses an exceptional mutant ability -- such as a 50-foot tongue lasso or a giant belly full of explosive methane gas -- instead of traditional firearms.

Setting out to do for co-op gaming what Counter-Strike did for team combat, Left 4 Dead is an ambitious survival horror game. Running on the Source engine, Left 4 Dead leaves four armed survivors of a world overrun with zombies, and they must fight their way out to escape the outbreak.

Designed for co-op play, the four players must work together to finish each stage of the game, lending each other artillery support, sharing ammo and rescuing each other when zombies are on top of them. In addition to the standard four players as humans, another four players can be amongst the masses of assaulting zombies, seeking to take down the other players. Zombie players might even find themselves become one of the four mutant "boss" zombies, who have unique powers that can be used to devastate, injure, tie up, mark and track, confuse, or even humiliate the gun-toting human survivors.