All species of thecate and athecate hydroids (excl. Stylasteridae) from Greenland and Iceland are reviewed. The revision is based predominantly on new material collected during various expeditions, but also on types and material used by previous authors. It includes also samples from the north of Greenland (Pearyland), a locality from which no hydrozoans were known so far. These samples produced the northernmost records for hydroids (82°N): Obelia longissima, Lafoea dumosa, and Boreohydra simplex.
The genus Clava is transferred to the Family Hydractiniidae. The name Cordylophoridae Lendenfeld, 1885 is used for the remaining species formerly held in the same family as Clava. The gonophores of Halisiphonia arctica are free medusae. Halisiphonia is therefore removed from the Lafoeidae and referred to the family Hebellidae Fraser, 1912.
New records for Greenland are: Boreohydra simplex Westblad, 1937; the polyp phase of Euphysa aurata Forbes, 1848; Eudendrium islandicum Schuchert, 2000; Acaulis primarius Stimpson, 1854; Campanulina pumila (Clark, 1875); Sertularia schmidti Kudelin, 1914; Sertularia similis Clark 1877; Thuiaria articulata (Pallas, 1766); and Thuiaria sachalini Kudelin, 1914.
New records for Iceland are: Rhizogeton nudum Broch, 1903; Rhizorhagium roseum Sars, 1874; Hydractinia allmanii Bonnevie, 1898; Coryne loveni (M. Sars, 1846); Sarsia producta (Wright, 1858); Filellum serratum (Clarke, 1879); Halecium birulai Spassky, 1929; and Symplectoscyphus bathyalis Vervoort, 1972.