Multi-User Dungeon, or MUD (referred to as MUD1, to distinguish it from its successor, MUD2, and the MUD genre in general) is the first MUD and the oldest virtual world in existence. It was created in 1978 by Roy Trubshaw at Essex University on a DEC PDP-10 in the UK, using the MACRO-10 assembly language. He named the game Multi-User Dungeon, in tribute to the Dungeon variant of Zork, which Trubshaw had greatly enjoyed playing.Zork in turn was inspired by an older text-adventure game known as Colossal Cave Adventure or ADVENT.
In 1980 Roy Trubshaw created MUD version 3 in BCPL (the predecessor of C), to conserve memory and make the program easier to maintain.Richard Bartle, a fellow Essex student, contributed much work on the game database, introducing many of the locations and puzzles that survive to this day. Later that year Roy Trubshaw graduated from Essex University, handing over MUD to Richard Bartle, who continued developing the game. That same year, MUD1 became the first Internet multiplayer online role-playing game as Essex University connected its internal network to the ARPAnet.
In 1983 Essex University allowed remote access to its DEC-10 via British Telecom's Packet Switch Stream network between 2 am and 7 am each night. MUD became popular with players around the world, and several magazines wrote articles on this new trend.
In 1984 Roy Trubshaw and Richard Bartle were approached by book editor and gamer Simon Dally to form a company to promote and market MUD, and to produce and market the next generation of multi-user games. As a result Multi User Entertainment (MUSE) Ltd was formed in 1985. The late Dally also wrote competitions for Acorn User magazine. He told the editor Tony Quinn that he found the minicomputer to run the game in a builder's skip.
In 1985 Richard Bartle created MUD1 version 4, better known as MUD2. It was intended to be run as a service for British Telecom.
In 1987 MUD1 was licensed by CompuServe, who pressured Richard Bartle to close down the instance of MUD1, better known as 'Essex MUD', that was still running at Essex University. This resulted in the deletion of the MUD account in October 1987. This left MIST, a derivative of MUD1 with similar gameplay, as the only remaining MUD running on the Essex University network, becoming one of the first of its kind to attain broad popularity. MIST ran until the machine that hosted it, a PDP-10, was superseded in early 1991. 
MUD1 ran under the name British Legends until late 1999 and was retired along with other software during CompuServe's Y2K cleanup efforts.
In 2000 Viktor Toth rewrote the BCPL source code for MUD1 to C++ and opened it alongside MUD2 on British-legends.com.
- Kevin Kelly, Howard Rheingold (1993). "The Dragon Ate My Homework". Wired 1 (3). "In 1980, Roy Trubshaw, a British fan of the fantasy role-playing board game Dungeons and Dragons, wrote an electronic version of that game during his final undergraduate year at Essex College. The following year, his classmate Richard Bartle took over the game, expanding the number of potential players and their options for action. He called the game MUD (for Multi-User Dungeons), and put it onto the Internet."
- Richard Bartle (2003). Designing Virtual Worlds. New Riders. p. 741. ISBN 0-13-101816-7. "The "D" in MUD stands for "Dungeon" [...] because the version of ZORK Roy played was a Fortran port called DUNGEN."
- Tim Anderson; Stu Galley (1995). "The History of Zork". "Zork was too much of a nonsense word, not descriptive of the game, etc., etc., etc. Silly as it sounds, we eventually started calling it Dungeon. (Dave admits to suggesting the new name, but that's only a minor sin.) When Bob the lunatic released his FORTRAN version to the DEC users' group, that was the name he used."
- Richard Bartle (1990). "Early MUD History". "The program was also becoming unmanageable, as it was written in assembler. Hence, he rewrote everything in BCPL, starting late 1979 and working up to about Easter 1980. The finished product was the heart of the system which many people came to believe was the "original" MUD. In fact, it was version 3."
- Eddy Carroll (1995). "MUD Timeline". "Roy graduates from Essex University, and Richard takes full control of the game, fleshing out the database and adding additional commands. A proper persona communication system is introduced, along with the concepts of points and wizards."
- Mulligan, Jessica; Patrovsky, Bridgette (2003). Developing Online Games: An Insider's Guide. New Riders. p. 444. ISBN 1-59273-000-0. "1980 [...] Final version of MUD1 completed by Richard Bartle. Essex goes on the ARPANet, resulting in Internet MUDs!"
- Eddy Carroll (1995). "MUD Timeline". "Essex University allows outside users to access its DEC-10 via BT's Packet Switch Stream network (PSS) during the normally idle period from 2am to 7am each night."
- Richard Bartle (1995). "MUD Magazine Bibliography".
- Richard Bartle (2002). "MUSE background". "The beginnings of MUSE were in 1978, when Roy Trubshaw wrote the very first prototype of the computer game MUD. He and Richard Bartle created the bedrock upon which all subsequent MUDs were built, and such was the popularity of the game that in 1984 they were approached by book editor and games-player Simon Dally to form a company to promote and market MUD itself, while designing and implementing additional games. MUSE was formed in 1985, which makes it a comparative old-timer in the online games industry."
- Computer Magazines page at Magforum.com
- Richard Bartle (1999). "CompuNet MUD". "The incarnation of MUD1 on the CompuNet network in the UK, the first commercial MUA in the world."
- Richard Bartle (2002). "MUSE background". "A new version of the game, which came to be known as MUD2, was written in 1985 to be run as a service for British Telecom."
- Michael Lawrie (2003). "Escape from the Dungeon". "October of 1987 was chaos. The MUD account was deleted, but the guest account on Essex University remained open. I guess it wasn't causing any trouble so they simply left it. ROCK, UNI and MUD all ran from the MUD account so they had gone but... MIST ran from a student account and it was still playable."
- Richard Bartle (2007). "A Brief History". "Due in part to a fortuitous coincidence (MUD was written for the same DECSystem-10 computing platform that CompuServe used for its information service) MUD was licensed by CompuServe in the mid-1980s where it ran as a popular game until late 1999. It was eventually retired along with other software during CompuServe's Y2K cleanup efforts."
- Richard Bartle (2002). "Incarnations of MUD". "Viktor Toth had had a copy of the BCPL source code for MUD1 for some years, and decided that now was the time to do something with it. In a 9-day programming blitz over Christmas, he rewrote the BCPL MUDDL engine in C++ and opened it up alongside MUD2. The ex-CompuServe players gravitated there, where it now runs as a direct continuation of the defunct original BL incarnation."