Genestealers are a sub-species of Tyranid tasked with infiltrating the communities of other species and preparing them for Tyranid invasion. To achieve this goal, Genestealers will seek out and infect victims with Tyranid genetic material, bending their will to the hive mind and compelling them to reproduce. Succesful procreation of these infected victims will invariably result in the birth of a hybrid of Tyranid and the species of the infected host.

The first Genestealer to arrive on a targeted planet is known as the Broodlord and is considered the strongest of it

As new generations of Genestealer hybrids are born, each generation gradually appears closer in appearance to their host species; while the original Genestealer responsible for this "family", now referred to as a Genestealer cult, becomes known as a Patriarch. (The relationship between the Broodlord and Patriarch remains unclear).

The Patriarch, regarded as a father to hybrid cultists, eventually develops psionic powers which grows in accordance with the population of the Genestealer cult. While the cult infiltrates and even assumes command of planetary organisations of the host species on the planet, including the local government, the Patriarch gradually becomes a living psychic beacon in order to summon the Tyranid hive fleets to consume the planet. If the cult and Patriarch are not exposed and exterminated before this happens, the Tyranids will be summoned to the planet and resistance against the invasion will be hindered by the cultists revealing their true nature and actively sabotaging defence efforts.

Although Genestealer cults share a localised hive mind governed by the Patriarch, they typically share a religious perspective on the Tyranids, worshipping the complete hive mind as a god. After the Tyranid invasion begins, the Patriarch's influence is immediately superseded by the hive mind and the hybrid cultists are devoured with all viable material on the planet.

Although Genestealers typically infect Humans, due to their widespread population across the galaxy and ease to inflitrate, there have been a number of incidents in which other species became subject to Genestealer hybridisation. For example, Orks have been infected by Genestealers on occasion, but rarely due to the difficulty in properly establishing a cult as Ork populations would be difficult to infiltrate. The Tau also reportedly struggled to contain an outbreak of Genestealer infection, although their requirement of authorisation from the Ethereal cast to start families would make full establishment of a Genestealer cult a long and difficult process in comparison to Human infiltration. Due to their highly developed psychic abilities, Genestealer infiltration within Eldar societies is largely considered impossible. However; an Eldar craftworld was reportedly taken over entirely by Eldar-Genestealer hybrids. Interestingly, the Avatar of Khaine residing on this craftworld has reportedly been mutated by the Genestealer infestation, despite being a dormant manifestation of a god.

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