It is written in C and originally used the Allegro library.
It was originally released July 17th, 1999.
The latest version of the engine, MUGEN 1.1b (Mugen 1.1 Beta release) now uses the SDL library.
Beta versions were made for DOS, Linux and Windows, and was distributed through their website.
In fact, they admittedly stated that they forgot what it stood for. ( It is also known as Mugen, MUGEN, and M.U.G.E.N)
The meaning of "mugen" (無限) in Japanese, "unlimited" or "infinite", may have also influenced the naming.
M.U.G.E.N allows users to create their own 2D fighting game using the core game engine provided. From the playable characters, to the stages, to the background music, to the life bars & power bars, text font, and even in-game character stories.
Creating content for mugen is a time-consuming process (as it is for a real fighting game developer).
Content is primarily created by using ripped sprites from commercial 2D games, and coding them via the Mugen C programming language as playable content in Mugen. Creators then upload and share their creations among the community for others to download/critique/edit/use.
The Mugen community consists of Contributors or "creators" which congregate on various MUGEN websites such as Mugen Fighters Guild, Mugen Infantry, Mugen Free For All, and Mugen Archive.
M.U.G.E.N was initially created for MS-DOS.
Development of the DOS version ceased when Elecbyte switched to the Linux platform in November 2001.
For a time, Elecbyte had posted a request for donations on their site to legally obtain a Windows compiler to make a Windows version of M.U.G.E.N. However, the development group discontinued the project in 2003 and shut down their site.
Later speculation pointed at leaks made public of a private Windows-based M.U.G.E.N beta that was provided to a small quantity of donors.
The private WinM.U.G.E.N beta contained a two-character roster limit, locked game modes and nag screens.
With the beta leaked and Elecbyte gone, a "no limit" hack that removed most of these limitations was made available in 2004 followed by subsequent updates to deal with bugs and other issues.
This version of M.U.G.E.N is functionally the same as the last Linux release, though with subtle differences and unique issues mostly revolving around proper music and music plugin support.
Because of the changes between the DOS and Linux versions of M.U.G.E.N however, many older characters required at least the SFF files to be modified to show palettes correctly (notably on portraits) as well as some changes in how certain CNS script controllers functioned, causing some minor upset and those that could still run the DOS version in some form sticking to that, as well as DOS patches to downgrade characters to be compatible with the older version of the engine.
In May 2007, a hacked version of WinM.U.G.E.N was released by a third-party that added support for high resolution stages at the cost of losing support of standard resolution M.U.G.E.N stages.
Later that month, another hack added support for high-res select screens.
In July 2007 another hack based on the last high-res hack allowed for only the select screen to be high-res and not the stages.
In December 2007, a hack from an anonymous source allowed both low-res and hi-res stages to be functional in the same build.
As of June 2007, an unofficial Winmugen was also made available on a Japanese website.
In mid 2007, Elecbyte's site returned, though not without some controversy as to the legitimacy of it, as it only showed a single logo with Google ads on the side.
On July 26 a FAQ was added to the site, which went on to claim that they would release a fixed version of WinM.U.G.E.N before major format changes in the next version, and noted the formatting changes would remove compatibility in regards to older works: "Do not expect old characters to work. At all".
On September 19, 2009, Elecbyte made an unexpected comeback, updating their website with various features — including a forum and a downloads section, where a new build of M.U.G.E.N was available.
In September 2009, a full release of M.U.G.E.N (MUGEN 1.0 Release Candidate) that includes various new features — most notably (official) support for HD resolutions, victory screens and language localization — was made available through the Elecbyte website.
Although this build had various visual glitches and required a fair amount of adjustments to the previously made content in order to be fully compatible with the new engine, Elecbyte has stated that it is their goal to have the new M.U.G.E.N fully compatible with previously designed content.
On January 18, 2011, Elecbyte released a 1.0 version only for Windows, ironing out most bugs that were featured in the release candidates.
It was compatible with almost all, if not all, of the previously made content.
After the 1.0 release, Elecbyte again ceased public activity for an extended time.
In late April/early May 2013, a leaked copy of M.U.G.E.N 1.1 alpha 4 hit the Internet. This version added stage-zooming capabilities and some other features, although as expected with an alpha, it had numerous bugs. A post was made on the website on May 11, 2013 regarding the forums having problems. However, as of May 28, 2013, the forums are back online.
In August 2013, M.U.G.E.N 1.1 beta 1 was released to the public, which fixes many of the bugs from the 1.1 alpha versions. Additional releases for 1.1 were planned and currently being worked on.
These releases were planned to include significant engine changes that would remove certain character development constraints that existed due to limitations of the old code.
On July 8, 2014, a fan-made port of M.U.G.E.N 1.0 for Mac OS X was released by Mugenformac, built using the "Wineskin Winery" wrapper.
It ran with few to no port-related issues.
Version 1.1 beta 1 was then released on January 3, 2015. As of September 2015, Elecbyte's site is down once again for unknown reasons; nowadays, it holds a 403 message.
M.U.G.E.N. gained mainstream press with the creation of the Twitch live stream called Salty's Dream Cast Casino (Salty Bet), where viewers can bet with fake money on CPU matches played using the engine.
Although there a few public figures in the mugen community, Youtube channels with public personalities such as MugenLord, Alexlexus(NMO), Swagga Kings, Immortal syn, 2dee4ever, have helped popularize the engine among more mainstream audiences.
Mugen was also streamed live by Famous Youtuber/FGC figure Maximillian Dood on several occasions and featured on his channel.