id Tech 4, popularly known as the Doom 3 engine, is a game engine developed by id Software and first used in the video game Doom 3. The engine was designed by John Carmack, who also created previous game engines, such as those for Doom and Quake, which are widely recognized as significant advances in the field. This OpenGL-based game engine has also been used in Quake 4, Prey, Enemy Territory: Quake Wars, Wolfenstein, and Brink. id Tech 4 is licensed under the terms of the GNU General Public License v3.0 or later.

A game engine is a software framework primarily designed for the development of video games and generally includes relevant libraries and support programs such as a level editor. The "engine" terminology is akin to the term "software engine" used more widely in the software industry.