--> "A weekend hack that got out of hand."
-->--'''James Aspnes''', describing [=TinyMUD=]

Multi-User Dungeons, or {{MUD}}s for short, are essentially the forerunners to modern {{MMORPG}}s. Their primary distinguishing feature is that they utilize a text-based interface similar to InteractiveFiction games, but still allow dozens or even thousands of users to play at the same time. Interestingly, [[VideoGame/MultiUserDungeon the first MUD]] was so named because it was designed as a multiplayer version of ''Dungeon'' (an early version of ''VideoGame/{{Zork}}'') making it ExactlyWhatItSaysOnTheTin twice over.

Despite the current popularity of {{MMORPG}}s with flashy extra features like graphics, [=MUDs=] are still popular amongst certain groups, since they are relatively easy to maintain, require little to nothing in the way of installed programs, and are almost always free to play.

A MUD is controlled by inputting typed commands, and is divided into undefined areas called rooms. You move by inputting a directional command ('north', 'west', 'southeast', or 'n', 'w' 'se'); attack by ordering yourself to kill ('kill bear' or 'k bea'); sleep by entering 'sleep' (or 'sl') etc., with most commands allowing some degree of abbreviation.

Since the designers don't have to worry about rendering images or sounds for their content (most objects, creatures, and effects are described by a few lines of text and/or code), [=MUDs=] can get extremely in-depth, ''and'' extremely complicated. Expect different effects for eating, drinking, sleeping, resting, hiding, having a wound on a certain body part, being blinded, stunned, underwater, in the vacuum of space, in the faerie dimension, flying, or just tired. The ability to forge your own weapons, speak different languages, buy or build your own house, marry another player, declare another player attack-on-sight, worship different gods, and start your own guild are all fairly standard. The emotes list can often get into the hundreds, and if none of them suits, you can even improvise with custom emotes. And that is not even getting into the combat skills, spells, proficiencies and other statistics you can learn and improve upon.

Variants include the more free-form {{MUSH}}, often called a [[MultiUserSharedHallucination Multi-User Shared Hallucination]], which is a kind of [=MUD=] that is mostly used as a roleplaying environment or sometimes, a set of chat rooms; along with the traditional roleplaying-oriented {{MUCK}} and the {{MOO}}. These are descended from [=TinyMUD=] and forego the extensive, automated combat systems of most [=MUDs=] in favour of either free-form roleplay, system-supported/[[TabletopRPG Pen & Paper style roleplay]], or some other specific format. The term MUD has itself been [[FunWithAcronyms [=backronymed=]]] into "Multi-User Dimension", "Multi-User Domain", and several other meanings, possibly as a way to distinguish the variants that are not intended as games, or that have a ScienceFiction theme instead of a {{Fantasy}} theme.

[[http://www.mudconnect.com/ The Mud Connector]] is a portal for finding, reviewing, and discussing [=MUDs=], with a convenient Flash-based telnet client available for use. There are over a thousand [=MUDs=] of many genres available to browse. [[http://mudstats.com/ MUDStats.com]] also lists several hundred [=MUDs=] by type, genre, and activity level.

Compare InteractiveFiction, aka TextAdventure, which are script-oriented games. Contrast [[PointAndClickGame Point And Click]] {{Adventure Game}}s, more graphical scripted games which may also use a fixed-room format or TextParser.
!!Tropes common to [=MUDs=]:
* YouCantGetYeFlask - [=LPMud=] may have IF-style quests in which part of the puzzle is figuring out what can be interacted with and picking the correct syntax. Most other Mud types avert this; items that can be taken and features that can be interacted with are separated from room description by formatting, and sometimes color-coded.
* GuideDangIt: New players can find [=MUDs=] very frustrating at first, particularly when the "help" entry for what you are trying to do does not seem to exist. Asking other players is often the best solution, though this is sometimes discouraged or outright forbidden.
* WizardNeedsFoodBadly: Most characters will need to eat (and drink) regularly to keep their HitPoints and/or {{Mana}} at maximum, though procuring them is not usually difficult.
* MassiveRaceSelection: Since you don't have to render what a [[EldritchAbomination Yithian]] looks like while wearing a {{mithril}} hauberk and wielding an [[FamilyFriendlyFirearms energy rifle]], there is little reason why you can't let people play as one.
* An enormous range of {{Character Level}}s, often several hundred, if there even is a level cap. Can make finding a worthwhile-but-beatable foe difficult.
* A concomitant amount of LevelGrinding to go with those levels.

* ''Aardwolf''
* ''VideoGame/{{Achaea}}''
* ''VideoGame/{{Aetolia}}''
* ''AncientAnguish LPMUD''
* ''Videogame/ArmageddonMUD''
* ''Astaria''
* ''VideoGame/AtonementRPI''
* ''VideoGame/TheBurningPostII''
* ''VideoGame/CthulhuMUD''
* [[http://www.cleftofdimensions.net/ Cleft of Dimensions]]
* ''VideoGame/DarkAndShatteredLands''
* ''VideoGame/DiscworldMUD''
* ''VideoGame/EmpireMUD''
* ''VideoGame/TheEternalCity''
* ''FlexibleSurvival''
* ''Genesis LPMUD''
* ''[[http://www.gnsmud.org/forum/index.php/ GNS Mud]]''
* ''Godwars2''
* ''Icesus''
* ''VideoGame/InfernoMOO''
* ''VideoGame/TheInquisitionLegacy''
* ''LegendsOfTheJedi''
* ''VideoGame/LostSoulsMUD''
* ''VideoGame/{{Lusternia}}''
* ''MateriaMagica''
* ''MegaMUD''
* ''Miriani''
* ''VideoGame/MultiUserDungeon''
* ''VideoGame/NewWorldsAteraan''
* ''RiversOfMUD'' (ROM)
* ''VideoGame/RetroMUD''
* ''Sabrous''
* ''Sanity's Edge''
* ''ShatteredKingdoms''
* ''StarConquest''
* ''Thunderdome''
* ''VideoGame/UnwrittenLegends''
* ''{{UOSSMUD}}''
* ''VideoGame/TheWorldCalledHollow''