Living up to its double-meaning title, Left 4 Dead features four living playable characters who have been left behind to survive in a city with a rapidly growing population of people-eating walking dead. In classic horror-film style, the four main hero characters are largely dissimilar and not likely to get along very well at first, but they will need to learn to trust one another and work together to survive the streets of the rampaging zombies. Computer-controlled companions are available for single-players, but online multiplayer co-op games are a main thrust of play. In a standard game, players begin each level in a safe area, where they can stock up on supplies and put together a plan before moving out into the zombie-flooded city streets.

No place is safe for too long in this game, however, and once the zombies start coming, they keep coming, no matter where the players run or hide. Generally, players will take a role on their team that matches the weapons or medical equipment they chose at the beginning of the level. Friendly fire is painfully possible, so players will need to be careful where they aim and shoot. Sometimes, in close quarters, a quick melee attack may be a safer way to temporarily stave off encroaching undead or knock a hungry zombie off a companion. The ultimate objective in each level is not known from the start, but created in part by a built-in artificial intelligence "Director," which analyses the players' positions and progress, and creates new challenges on the fly, making each game different and adding creepy factor of the unknown.

Left 4 Dead was developed by Valve Software, built on the studio's Counter-Strike: Source game engine. In addition to the four-person survival-horror game, available modes of play allow multiplayer competition for up to eight, with one team of four playing as the human survivors and the other four players taking the roles of infected boss zombies. Each boss zombie has a different special power, such as a prehensile tongue or an explosive belly full of corrosive swamp gas, encouraging them to combine their strengths and work as a team, just like the poor human beings they are hunting down for supper. ~ T.J. Deci, All Game Guide