This atlas covers 85 species of water beetles in five main families (Hydraenidae: minute moss beetles; Elmidae: riffle beetles; Heteroceridae: variegated mud-loving beetles; Scirtidae: marsh beetles; Dryopidae; plus a small number of other wetland species in the suborder Polyphaga of the Coleoptera). This atlas is complementary to the previous two parts concerning the Hydradephaga and Hydrophiloidea. All three books are rather more than sets of maps, with as much effort made to identify gaps in the knowledge of their biology as to establish their distributions. The present recording scheme began in 1979. Over 550,000 records having been acquired through the activities of over 700 recorders, supported by the Biological Records Centre in Britain and the National Biodiversity Data Centre and CEDaR in Ireland. This section of the atlas draws on over 79,000 records (Hydraenidae 20,000; Dryopidae 4,500; Elmidae 42,100; Heteroceridae, 1,700; Scirtidae 10,700).