Tyranids were first described in Rogue Trader, the first edition of the Warhammer 40,000. At that time they were not an emphasized race in the game, instead representing a limited number of occasionally encountered alien antagonists. Their physical appearance was not imposing or especially horrific: they were depicted as six limbed, relatively diminutive creatures (an appearance which would later be assigned to Tyranid termagants). In later iterations of Warhammer 40,000-related products (starting with the release of Advanced Space Crusade) the Tyranids were given a complete makeover and became a major race, popularized by a number of successful expansions. Unlike most Warhammer 40,000 races, the Tyranids do not have a direct Warhammer Fantasy Battle counterpart, but share a horde mentality with the Skaven and Lizardmen armies while bearing some resemblance to the latter through having somewhat reptilian features. This, along with the extensive use of larger, stronger creatures and primal defense mechanisms leads to the widely-drawn inference that the Tyranids are the closest Warhammer 40,000 counterpart to the Lizardmen.
Tyranids often form a fast-moving close-combat army that relies on overwhelming its enemies through waves and waves of mostly small but deadly creatures. They may also rely heavily on ranged combat, if different selections are made, or field an impressive but small array of monstrous titans rather than a typical swarm. Tyranid armies are therefore able to bring a varied threat to bear on their opponents.