MUGEN is a 2D fighting game engine developed by Elecbyte. Content is created by the community, and thousands of fighters, both original and from popular fiction, have been created.

Often authors will port popular characters from 2D fighting engines such as the Street Fighter series, or from TV, book, and game series such as Teenage Mutant Ninja Turtles, The Simpsons, Pingu, Super Mario, Sonic The Hedgehog, Pokémon, Sailor Moon, and Touhou. Many authors will also create original content. Many websites exist to showcase and disperse the developed content and forms in what is often referred to as the "Mugen Community". Games that are built using the M. U. G. E. N engine often focus around a single franchise, such as Hyper Dragon Ball Z. Due to the customizable nature of the game engine, no two versions of M. U. G. E. N are the same. Each person is encouraged to download their own copy of the game engine and to create or add content to match their personal preference. Groups of M. U. G. E. N authors will often collaborate to produce a full game using the engine. These full games are available at a variety of quality levels and are released under the general M. U. G. E. N license. "Under this license, permission is granted to use the M. U. G. E. N Environment free of charge for non-commercial purposes... Elecbyte provides a M. U. G. E. N redistributable package, containing a minimal M. U. G. E. N Environment, that may be included with third party content for redistribution.