[1] Immediately after its release in 1993, Doom attracted a sizeable following of players who created their own mods for WAD files—packages containing new levels or graphics—and played a vital part in spawning the mod-making culture which is now commonplace for first-person shooter games.

Thousands of WADs have been created for Doom, ranging from single custom levels to full original games; most of these can be freely downloaded over the Internet.

John Carmack, lead programmer at id Software, designed the Doom internals from the ground up to allow players to extend the game.

[2] The idea of making Doom easily modifiable was primarily backed by Carmack, a well-known supporter of copyleft and the hacker ideal of people sharing and building upon each other's work, and by John Romero, who had hacked games in his youth and wanted to allow other gamers to do the same.

In contrast, some video game artists, including Jay Wilbur and Kevin Cloud, objected due to legal concerns and the belief that it would not be of any benefit to the company's business.

[3] Carmack additionally released the source code for the utilities used to create the game, but these were programmed in Objective-C, for NeXT workstations, and were therefore not directly usable by the mass userbase of IBM PC compatible.

Around 1994 and 1995, WADs were distributed primarily through BBSs and via CD collections found in computer shops or bundled together with instruction guides for level creation, while in later years Internet FTP servers became the primary method for obtaining these works.

During this time, id Software was working on their next game, Quake, using new technology, but started projects picking up the most talented WAD makers from the Doom community to create official expansions and to compete with the unauthorized collection CDs.

The team produced the 21 Master Levels, which, on December 26, 1995, were released on a CD along with Maximum Doom, a collection of 1,830 WADs that had been downloaded from the Internet.

With the source code available, it became possible for programmers to modify any aspect of the game, remove technical limitations and bugs, and add entirely new features.

The most common type of WAD consists of a single level, usually retaining the theme of the original game, but possibly including new music and some modified graphics to define a more distinctive setting or mood.

Freedoom is a project aiming to create a free replacement (modified BSD License) for the set of graphics, music, sound effects, and levels (and miscellaneous other resources) used by Doom.

[47] A similar project, Blasphemer, aims to create a complete free version of Heretic, but is less fully developed than Freedoom, from which it has recycled assets.