Chaosium is one of the longer lived publishers of role-playing games still in existence. Founded by Greg Stafford, its first game was a wargame, White Bear and Red Moon, which was later reworked to become Dragon Pass and its sequel, Nomad Gods. White Bear and Red Moon is notable for containing the first published material about Glorantha, later used as the primary setting for the role-playing games RuneQuest, Hero Wars and HeroQuest. Chaosium is the publisher of Call of Cthulhu, based on the stories of H.P. Lovecraft.
The rules of RuneQuest, Chaosium's first role-playing game, were distilled down into a generic, genre-neutral format known as Basic Role-Playing (BRP). These generic rules formed the basis of many of Chaosium's later RPGs, such as Call of Cthulhu, Stormbringer, Nephilim, and Ringworld.
Chaosium and Greg Stafford are also responsible for Pendragon, an Arthurian RPG now published by White Wolf, Inc.'s ArtHaus imprint after a spell with Green Knight Publishing. Other games of note include Thieves' World, Mythos, Elfquest, Worlds of Wonder, Superworld, Hawkmoon, Stormbringer, and Prince Valiant: The Story-Telling Game.
Several notable RPG authors have written material for Chaosium, including Steve Perrin, Sandy Petersen, Lynn Willis, Keith Herber, David Conyers, Ken St. Andre, Mike Mason, and Arduin creator David A. Hargrave.
Three magazines, until recently all of them defunct, had been published by Chaosium to promote its products:
- Wyrm's Footnotes ran for fourteen issues from 1976 to 1995. Initially it was a supporting material publication for White Bear and Red Moon but for its 11th issue, in 1981, it had become the official RuneQuest magazine. The magazine was revived in 2012 by Moon Design Publications, continuing the issue numbering at 15, despite the 17-year hiatus.
- Different Worlds. Forty-seven bimonthly issues from Different Worlds were published. Chaosium, from 1979 to 1985, published the thirty-eight first ones and Sleuth Publications, from 1985 to 1987, the nine last ones. In spite of being an old Chaosium's and Stafford's collaborator, Tadashi Ehara was the editor of the magazine during the periods concerned by both publishing houses.
- Starry Wisdom, a Lovecraft themed magazine, three issues of which Chaosium published in 1997.
Some Chaosium products have been translated into French, Portuguese, Japanese, German, Spanish and Italian, and were available in France from Jeux Descartes, in Germany from Pegasus Press, in Spain from Joc Internacional and La factoría de ideas and in Italy from Stratelibri and Grifo Edizioni.
In the mid-1980s, Chaosium entered into a complex arrangement with Avalon Hill to publish RuneQuest material while Chaosium maintained editorial control over Glorantha-based material for the game (which Avalon Hill would publish). While this agreement reportedly kept the company in existence, it also left RuneQuest moribund, leading to products of questionable quality, long gaps with no products published at all, and, eventually, the death of the game altogether.
In response to collectible card games' popularity after the emergence of Magic: The Gathering, Chaosium released the now discontinued Mythos CCG.
In the late 1990s, Chaosium effectively split up into various successor companies, each maintaining its focus on a few of the company's products. Green Knight Publishing formed to focus on Pendragon, Chaosium "proper" retained Call of Cthulhu, Stormbringer, and Mythos, while Greg Stafford (who left Chaosium in 1998) founded Issaries, Inc. to publish HeroQuest and focus on bringing new Glorantha related material into print. Also, Wizard's Attic (no longer in business) was formed in order to act as a fulfillment house.
Chaosium continues to release roleplaying supplements for its Call of Cthulhu and BRP product lines. In 2012, Cthulhu by Gaslight won two Ennie awards at GenCon Indian;, Gold for Best Supplement and Silver for Best Cover Art. Later that year Cthulhu by Gaslight also won the Diehard GameFAN award for Best Re-Make/Re-release. Mythic Iceland by Pedro Ziviani won the Diehard GameFan award for Best Campaign Setting. Chaosium successfully funded a kickstarter for a re-release of the box-set campaign, Horror on the Orient Express. In 2013 Chaosium returned to Fantasy Roleplaying with its release of a new edition of Magic World.
In June of 2015, Chaosium announced the return of both Greg Stafford and Sandy Peterson to the the firm. According to their announcement:
Greg Stafford, founder of Chaosium and creative force during its heyday, is now President. Sandy Petersen, world renowned game designer who brought Cthulhu into the light three decades ago, has rejoined the team as well. Greg says: "Chaosium is part of my legacy. My intent is to restore it to its rightful place in the world of gaming. Something we can all take pride in, and something that fans will look forward to. Where 'what’s next?' is answered with 'I can’t wait'." 
- ↑ Glorantha Magazine Indices. Erzo.org. Retrieved on 18 December 2014.
- ↑ Different Worlds Magazine Cover Listing - RPGnet RPG Game Index. Index.rpg.net. Retrieved on 18 December 2014.
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- ↑ Congratulations to the 2012 ENnie Award winners!. Ennie-awards.com. Retrieved on 18 December 2014.
- ↑ Diehard GameFAN - Diehard GameFAN’s 2012 Tabletop Gaming Awards!. Diehard GameFAN. Retrieved on 18 December 2014.
- ↑ Horror on the Orient Express: Call of Cthulhu by Chaosium. Kickstarter. Retrieved on 18 December 2014.
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- ↑ Error on call to Template:cite web: Parameters url and title must be specified. Retrieved on 11 June 2015.