The Hydrophiloidea, comprising the families Hydraenidae, Spercheidae, Hydrochidae, Georissidae, and Hydrophilidae, of Denmark and Fennoscandia are monographed. The 118 species hitherto known from the region, and 18 additional species from the adjacent areas, are included. All taxa are keyed, described, and illustrated. Notes on the biology and distribution are given. Ochthebius kaninensis Poppius, 1909 is considered a valid species, distinct from O. bicolon Germar, 1824. The name Pseudenochrus Lomnicki, 1911 is considered a synonym of Lumetus Zaitzev, 1908.
Type-species are designated for the following names: Hymenodes Mulsant, 1844: Type-species: Ochthebius pellucidus Mulsant, 1844 (= O. nanus Stephens, 1829). Bilimneus Rey, 1883: Type-species: Hydrophilus atomus Duftschmid, 1805. Enoplurus Hope, 1838: Type-species: Hydrophilus spinosus Steven, 1808.