Illustrated keys are provided for the families,genera, species and other taxa of British and IrishHydradephaga. The Hydradephaga comprise theGyrinidae, Haliplidae, Noteridae, Paelobiidae (alsoknown as Hygrobiidae) and the Dytiscidae. Colourplates are provided of all the species. Notes includecharacters for distinguishing the sexes, some basicinformation on biology and collecting methods,and reviews of distributions in Ireland and Britain,including the Channel Isles.