Source Engine is a 3D game engine developed by Valve in 2004 as a successor to the GoldSrc engine, providing support for rendering, sound effects, animation, anti-aliasing, interfaces, networking, art creation and physical simulation.

The first game made with this engine was Half-Life 2. Valve also used it to make better versions of Half-Life and Counter-Strike shortly after the release of Half-Life 2. Other games that use it include Half-Life 2: Deathmatch and Victory Day: Source.

In early 2005, Troika Games announced that their game Vampire: The Masquerade – Bloodlines uses the Source engine, and they were the first company to obtain a license to use it. Valve later announced that Arkane Studios (the company that made the first-person role-playing game Dark Messiah of Might and Magic) and Smiling Gator Productions had also obtained a license to use it.