Greyhawk is a supplementary rulebook written by Gary Gygax and Robert J. Kuntz for the original edition of the Dungeons & Dragons fantasy role-playing game. It has been called "the first and most important supplement" to the original D&D rules. By adding a combat system, it severed all ties with Chainmail, making D&D a truly stand-alone game system. Although the name of the book was taken from the home campaign supervised by Gygax and Kuntz based on Gygax's imagined Castle Greyhawk and the lands surrounding it, Greyhawk did not give any details of the castle or the campaign world; instead, it explained the rules that Gygax and Kuntz used in their home campaign, and introduced a number of character classes, spells, concepts and monsters used in all subsequent editions of D&D.
The original rules for Dungeons & Dragons were published by TSR in 1974, but were limited in scope: the character classes and monsters listed were small in number; and for combat rules, players needed to have a copy of Chainmail, a rulebook for miniatures wargames published by Guidon Games in 1971. Over the next two years, TSR bolstered the original rules with five supplemental books. Greyhawk was the first of these supplements, named after Gary Gygax's home campaign.
The 2004 publication 30 Years of Adventure: A Celebration of Dungeons & Dragons suggested that details of Gygax's Greyhawk campaign were published in this booklet. However Gygax had no plans in 1975 to publish details of the Greyhawk world, since he believed that new players of Dungeons & Dragons would rather create their own worlds than use someone else's. In addition, he didn't want to publish all the material he had created for his players; he thought he would be unlikely to recoup a fair investment for the thousands of hours he had spent on it, and since his secrets would be revealed to his players, he would be forced to recreate a new world for them afterward. In fact the only two references to the Greyhawk campaign were an illustration of a large stone head in a dungeon corridor titled The Great Stone Face, Enigma of Greyhawk and mention of a fountain on the second level of the dungeons that continuously issued an endless number of snakes.
The 68-page supplement instead focused on new game rules that had been developed by Gygax and Kuntz during long hours of home play. The supplement also removed the game's dependency on the Chainmail rules by providing its own set of combat rules, which made it much easier for new, non-wargaming players to grasp the concepts of play.
Greyhawk also introduced new character classes (thief and paladin), as well as new combat rules, spells, monsters, and treasures. Greyhawk included new rules on weapon damage varying by weapon. The supplement added new treasure and magic items, and new spells, including 7th, 8th, and 9th level spells. The supplement also included a section on monsters, introducing the lizard men, beholders, displacer beasts, blink dogs, carrion crawlers, and many others.
Greyhawk was already in process at the time of TSR co-founder Don Kaye's death in January 1975, and was published a few months later in the spring of 1975. It was designated Supplement I and given a product designation of TSR 2003, Many of the new rules presented in the supplement eventually became standard parts of the AD&D game. A second supplement, Blackmoor, followed later the same year.
Illustrations for the supplement were provided by Greg Bell.
Judges Guild's Ready Ref Sheets (1977) collected together 20 pages of charts from the original D&D box, Chainmail, and Greyhawk. Material from Greyhawk, along with the original D&D and the Blackmoor and Eldritch Wizardry supplements, was revised for J. Eric Holmes' Dungeons & Dragons Basic Set (1977).
In 2013, the Greyhawk supplement was reproduced as a premium reprint of the original "White Box" D&D rules. Each booklet features new cover art but otherwise is a faithful reproduction of the original.
Lawrence Schick, in his 1991 book Heroic Worlds, calls Greyhawk "The first and most important supplement to Original D&D".
Shannon Appelcline, in his 2011 book Designers & Dragons, considers Greyhawk an "innovation" because at the time "supplements were largely unheard of in the wargaming industry. Though games were frequently revised and reprinted, continually expanding a game was something new."