Illustrated keys are provided for the families, genera, species and other taxa of British and Irish Hydradephaga. The Hydradephaga comprise the Gyrinidae, Haliplidae, Noteridae, Paelobiidae (also known as Hygrobiidae) and the Dytiscidae. Colour plates are provided of all the species. Notes include characters for distinguishing the sexes, some basic information on biology and collecting methods, and reviews of distributions in Ireland and Britain, including the Channel Isles.