Source is a 3D game engine developed by Valve. It debuted in June 2004 with Counter-Strike: Source and shortly thereafter Half-Life 2. Other games using the engine include the physics sandbox Garry's Mod, first-person action game Dark Messiah of Might and Magic and the first-person beat-em-up Zeno Clash.

Alien Swarm Source came out 2010, Alien Swarm Reactive Drop was released 7 years later.