Genestealers (Corporaptor Hominii) are an alien species in the Warhammer 40,000 universe, and the opponents of the marines in the Space Hulk board game. In more recent versions of that universe, Genestealers are a biological subgroup of the Tyranids, a nomadic alien race comprising many genetically engineered forms.
In 4th Edition of the wargame, there are two subgroups of the Genestealers: the Broodlord and the Genestealer. In the 2nd Edition of the game, there are five subgroups, representing the Genestealer Cult variant army: Genestealer Patriarchs, Genestealer Magus, Genestealers, Genestealer Hybrids, and Brood Brothers. For information concerning biomorphs and other mutable genetic characters of the Genestealers in the game, see Tyranid.
Genestealers appeared in the first edition of Warhammer 40,000 (Rogue Trader), but at the time were not related to Tyranids. They were simply noted as an "enigmatic monster ... from one of the moons of Ymgarl", now "spread throughout space" and "threatening to become a real menace". Since then it has been revealed that they are the advance forces of a Tyranid invasion and are now often seen as part of a Tyranid army.
The Genestealer Cult is a sub army that has been introduced several times for Warhammer 40,000. The initial introduction by Paul Murphy, Bryan Ansell, and Nigel Stillman was printed in White Dwarf issues 114-116 (1989). Original inspiration for the idea came from a novel by Harris Moir. This book was followed by an article from Andy Chambers and Jervis Johnson in White Dwarf issue 145 (1992) that detailed the Genestealer Cult as part of a Tyranid Army. Later, Tim Huckelbery detailed the Genestealer Cult Army in issues 40 & 41 of the Citadel Journal, under the title Codex: Genestealer Cults. This official supplemental Codex provided official rules for using Patriarchs, Magi, Hybrids and Broodbrother units. It even included rules for a "Transport Coven Limousine." Later, in the first Codex Tyranids, these rules were incorporated as the "Genestealer Cult Army List" and included all of the characters in their previous form sans Limousine.