Ultima Online (UO) is a graphical massively multiplayer online role-playing game (MMORPG), released on September 25, 1997, by Origin Systems. It was instrumental to the development of the genre, and is still running today. The game is played online, in a fantasy setting similar to that of the other Ultima games that preceded it. The success of Ultima Online opened the door for the creation of many new massively multiplayer games. Ultima Online is a fantasy role-playing game set in the Ultima universe. It is online-only and played by thousands of simultaneous users (who pay a monthly fee) on various game servers, also known as shards. It is known for its extensive timing-based player versus player combat system. To maintain order in the online community, there are Game Masters who resolve player disputes, police the shard for terms of service violations, and correct glitches in the game.

Several expansions have been released, but its aging game engine and graphics make it outdated compared to competitive, new massively multiplayer games. The release of Ultima Online: Kingdom Reborn in 2007 brought a new game engine which offers a newer visual experience. Since Ultima Online`s prime in 2003, the overall subscriber base has seen a steady decline. Subscriber numbers peaked at around 250,000 in July 2003, and to date sit around 135,000 subscribers (approximately 70,000 of whom are Japanese). As of June 2006, Ultima Online held a 1.1% market share of the massively multiplayer online game subscriptions.

Quoting directly from the Electronic Arts press release announcing the Ultima Online: Kingdom Reborn expansion, Ultima Online was "The first MMORPG to reach the 100,000 subscriber base, far exceeding that of any game that went before it". Since then, it has added seven expansion packs and dozens of free content updates.

Ultima Online has seen many major revisions throughout its history. This includes gameplay revisions, staff changes, technical revamps, porting the backend to Unix, and fundamental design changes. With few earlier MMORPGs to take lesson from, the staff behind Ultima Online was breaking new ground and had to solve complex issues that had never been faced in a commercial game on such a wide scale before. The importance of understanding psychology, social interaction, economy, and other issues became increasingly important as complex social behavior evolved. Throughout the pre-release development of the game, a well-balanced, realistic economy and social structure was the goal. While not all of the features planned for incorporation made it into the first release, the developers did manage to give almost all of the control to the players in terms of what they could do to each other and the world as a whole. What ensued caused permanent repercussions still faced in the game today. Another problem throughout Ultima Online`s history has been flaws that allowed for cheating. In early years, methods to duplicate items were discovered and many took advantage of this loophole to mass produce gold and items, causing great harm in the game`s economy and power structure. Even after this method was fixed, other methods were discovered over the years which bypassed server and game mechanics to duplicate items and gold. With the introduction of cross-shard character transfers, massive duplication between worldwide servers started to occur, greatly injuring the game economy.

Ultima Online was sued by former player volunteers ("Counselors") and settled in 2004 without admitting wrongdoing.[9] AOL had their volunteers train customer service personnel it hired, then shut down the volunteer program. Concern over future lawsuits led Microsoft to shut down their volunteer program for Asheron`s Call. Bugs have also plagued the NPC monsters of Ultima Online; one such `bug` involved a particular `slime` monster that was designed to `replicate` into several smaller (identical) slimes when attacked by the players. While the replication worked as designed, it was soon discovered that using any of the game`s `area affect` spells triggered the multiplication in both the `original` slime and any `new` slimes that had been spawned, which caused the slimes to multiply exponentially until the server crashed under the strain.