A massively multiplayer online game (also called MMO and MMOG) is a multiplayer video game which is capable of supporting hundreds or thousands of players simultaneously. By necessity, they are played on the Internet, and feature at least one persistent world. They are, however, not necessarily games played on personal computers. Most of the newer game consoles, including the PSP, PlayStation 3, Xbox 360, Nintendo DSi and Wii can access the Internet and may therefore run MMO games. Additionally, mobile devices and smartphones based on such operating systems as Android, iOS, webOS, and Windows Mobile are seeing an increase in the number of MMO games available.

MMOGs can enable players to cooperate and compete with each other on a large scale, and sometimes to interact meaningfully with people around the world. They include a variety of gameplay types, representing many video game genres.

The first graphical MMOG, and a major milestone in the creation of the genre, was the multi-player flight combat simulation game Air Warriorby Kesmai on the GEnie online service, which first appeared in 1986.

Commercial MMORPGs gained early acceptance in the late 1980s and early 1990s. The genre was pioneered by the GemStone series on GEnie, also created by Kesmai, and Neverwinter Nights, the first such game to include graphics, which debuted on AOL in 1991.

As computer game developers applied MMOG ideas to other computer and video game genres, new acronyms started to develop, such asMMORTS. MMOG emerged as a generic term to cover this growing class of games. These games became so popular that a magazine, called Massive Online Gaming, released an issue in October 2002 hoping to cover MMOG topics exclusively, but it never released its second issue.

The debuts of The Realm Online, Meridian 59 (the first 3D MMOG), Ultima Online, Underlight and EverQuest in the late 1990s popularized the MMORPG genre. The growth in technology meant that where Neverwinter Nights in 1991 had been limited to 50 simultaneous players (a number that grew to 500 by 1995), by the year 2000 a multitude of MMORPGs were each serving thousands of simultaneous players and in December 2007 Eve Online achieved a new record with 41,690 concurrent accounts logged on to the same server. The current record stands at 60,453 achieved on 2010-06-06.

Despite the genre's focus on multiplayer gaming, AI-controlled characters are still common. NPCs and mobs who give out quests or serve as opponents are typical in MMORPGs. AI-controlled characters are not as common in action-based MMOGs.

The popularity of MMOGs was mostly restricted to the computer game market until the sixth-generation consoles, with the launch of Phantasy Star Online on Dreamcast and the emergence and growth of online service Xbox Live. There have been a number of console MMOGs, including EverQuest Online Adventures (PlayStation 2), and the multiconsole Final Fantasy XI. On PCs, the MMOG market has always been dominated by successful fantasy MMORPGs.

MMOGs have only recently begun to break into the mobile phone market. The first, Samurai Romanesque set in feudal Japan, was released in 2001 on NTT DoCoMo's iMode network in Japan. More recent developments are CipSoft's TibiaME and Biting Bit's MicroMonster which features online and bluetooth multiplayer gaming. SmartCell Technology is in development of Shadow of Legend, which will allow gamers to continue their game on their mobile device when away from their PC.

Science fiction has also been a popular theme, featuring games such as Anarchy Online, Eve Online, Star Wars Galaxies and The Matrix Online.

MMOGs emerged from the hard-core gamer community to the mainstream strongly in December 2003 with an analysis in the Financial Times measuring the value of the virtual property in the then-largest MMOG, Everquest, to result in a per-capita GDP of 2,266 dollars which would have placed the virtual world of Everquest as the 77th wealthiest nation, on par with Croatia, Ecuador, Tunisia or Vietnam.

Happy Farm is the most popular MMOG with 228 million active users, and 23 million daily users (daily active users logging onto the game with a 24-hour period).

World of Warcraft is currently a dominant MMOG in the world with more than 60% of the subscribing player base, and with 11–12 million monthly subscribers worldwide, is the most popular Western title among MMOGs. In 2008, Western consumer spending on World of Warcraft represented a 58% share of the Western subscription MMOG market. The title has generated over $2.2 billion in cumulative Western consumer spending on subscriptions since 2005.