Valve was founded as an L.L.C. based in Kirkland, Washington. After incorporation in 2003, it moved from its original location to Bellevue, Washington, the same city in which their original publisher, Sierra On-Line, Inc., was based.

After the success of Half-Life, the team worked on mods, spin-offs, and sequels, including Half-Life 2. All current Valve games are built on its Source engine, which owes much of its success to mods and sequels. The company has produced six games series: Half-Life, Team Fortress, Portal, Counter-Strike, Left 4 Dead, and Day of Defeat. Valve is noted for its support of its games' modding community: most prominently, Counter-Strike, Team Fortress, and Day of Defeat each began as a third-party mod that Valve purchased and developed into a full game. They also distribute community mods on Steam.