"We have been seeing very strong growth with Steam and Source, our content distribution and development platforms, up over 150% over the last 12 months. Given our expectations for Left 4 Dead and our long-standing relationships with members of the Turtle Rock team, this was an easy decision. It also gives us a base from which to expand our development activities in the Los Angeles area," said Gabe Newell, president of Valve.

"Left 4 Dead fills a long-standing demand gamers have had for a coop first-person action experience, and it will also help Valve's expansion into the console market," added Doug Lombardi, Valve's VP of marketing.

"Valve has had great success bringing in projects and teams such as Team Fortress and Portal. As I spent time working and talking with the creators of those products over the past several years, it became clear that this was the right next step for myself, my colleagues at Turtle Rock, our customers, and our products," said Michael Booth, founder and CEO of Turtle Rock Studios. "We are excited to be a part of such a visionary and forward-thinking company."