The Freedoom project aims to create a complete, free content first person shooter game, but Freedoom by itself is just the raw material for a game. It must be paired with a compatible Doom engine to be played.

There is a massive back catalog, spanning over two decades, containing thousands of Doom levels and other modifications (“mods”) made by fans of the game. Freedoom aims to be compatible with these and allows most to be played without the need to use non-free software.

Freedoom is actually three games in one, consisting of two single-player oriented campaigns and one set of levels designed exclusively for multiplayer deathmatch: