The distribution of native and alien crayfish inhabiting the British Isles is described separately for England, Wales, Scotland, Northern Ireland, Irish Republic, and some associated islands. Distribution of the single native species, Austropotamobius pallipes, is largely determined by geology, water quality and human intervention. Very large populations exist in England, Wales, and the Irish Republic, but in recent years some of these have been eliminated, particularly by crayfish plague in the 1980s. Active measures are being taken to conserve the remaining stocks of this species in line with the terms of the Rio Convention and European Directives. Since the 1970s, a number of alien crayfish have been introduced into England, Wales, and Scotland. Pacifastacus leniusculus is cultivated in a number of extensive and semi-intensive facilities. Astacus leptodactylus is not cultivated to any extent but those caught from the wild are used for culinary purposes. Large populations of both species now exist in the wild as a result of escapes and implants, particularly in central and southern England. The wild harvest of these two species is now probably greater than that derived from cultivation. Other alien crayfish such as Astacus astacus, Procambarus clarkii, and Orconectes limosus are also present in the wild in England, and Cherax quadricarinatus is kept by aquarists. Due to the environmental impact that wild populations of alien crayfish are having in Great Britain, stringent legislation has recently been introduced to try and stem their spread and that of crayfish plague. Northern Ireland and the Irish Republic ban the import of alien crayfish, although Great Britain does not. Reintroduction programs for A. pallipes are being initiated and monitoring of populations of both native and alien crayfish are being carried out at national and local level.