Left 4 Dead is a co-operative multiplayer action game from the makers of Counter-Strike.

"By leveraging its industry-leading AI technology and years of Counter-Strike development experience, Turtle Rock is building upon the elements that make Counter-Strike successful and applying them to Left 4 Dead," said Gabe Newell, co-founder and president of Valve. "What Counter-Strike did for multiplayer action games, Left 4 Dead will do for co-op games."

Left 4 Dead is built atop the most advanced version of the Source Engine and introduces version 2 of Turtle Rock's artificial intelligence (A.I.) technology.
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