id Tech is a series of separate
game engines designed and developed by
id Software. Prior to the presentation of the
id Tech 5-based game
Rage, the engines lacked official designation and as such were simply referred to as the
Doom and
Quake engines, from the name of the main game series the engines have been developed for. "id Tech" numbers 2, 3, and 4 have been released as
free software under the
GNU General Public License, along with the source code to
Wolfenstein 3D,
Doom, and
Quake.