Set in a modern day survival-horror universe, the co-operative gameplay of L4D casts four "Survivors" in an epic struggle against hordes of swarming zombies and terrifying "Boss Infected" mutants.

Valve's next generation AI technology, allows for games of 1 to 8 players. Players can play as a Survivor or as one of four types of "Boss Infected," each of whom possess a unique mutant ability such as a 50-foot tongue lasso or a giant belly full of explosive methane gas.

The gameplay takes place across four massive campaigns. The "zombie" population of each mission is choreographed by an "AI Director" that monitors the human players' actions and creates a unique and dramatic experience for them on the fly.