Development process:

Unreal Tournament 2004 was created by several game development studios acting in collaboration, including Epic Games who led the project. Lead programmer Steve Polge describes the role each company played:

Epic Games: All the enhancements to the Unreal Tournament 2003 game types, the new UI, Voice over IP and bot voice command support, engine enhancements and optimizations were made by the primary developer. They also made an improved single player game, and improved community and demo recording support, in addition to thirty-one new playable characters. A Sniper Rifle similar to the one included in the original Unreal Tournament was added. They created one Onslaught map, and developed AI support for Onslaught. 16 new DM maps, 5 new CTF Maps, 2 new DDOM maps, and 1 new BR map were added. The Assault game type design and implementation, including 3 new vehicles, 3 types of turrets, and 6 new maps were also reintroduced from the original Unreal Tournament, totaling 31 all-new maps from Epic.

The elimination and restoration of the Sniper Rifle and the Assault were noted on the official future history of the Tournament. The future history, written after the release of Unreal Tournament 2003 as promotional material for UT2004, reflected some real-life fan disappointment (including refusing to play the game) at the loss of these two things. The aging Sniper Rifle (a relic of centuries past) is removed from the Tournament as is "Assault" ... Many fans of the Tournament complain at these changes, with some combatants refusing to participate in the new format.An updated Sniper Rifle is added to the weapon roster, along with several other weapons designed for the larger outdoor combat environments of the new Onslaught and returning Assault events.

Psyonix: The Onslaught game type design and implementation, with 6 new vehicles, 4 new weapons (grenade launcher, spider mine layer, Anti Vehicle Rocket Launcher (AVRiL), and the Phoenix target painter), and the energy turret. They also created seven Onslaught maps, and collaborated with Streamline Studios on the popular map ONS-Torlan. Finally, they made the new model for the Translocator.

Digital Extremes: Created 2 new DM maps, 6 new CTF maps, 2 new BR maps, and 3 new DDOM maps; two new playable characters, the new HUD design; new weapon models for the Assault Rifle, Shock Rifle, and Link Gun.

Streamline Studios: The single player introduction movie and ONS-Torlan in collaboration with Psyonix. Later made an Assault map called AS-Confexia, downloadable from the Internet.