Characters from adult morphology are analyzed cladistically to infer the phylogeny of the family Dytiscidae. The analysis is based on examination of 233 species of Dytiscidae and several outgroup taxa including members of Noteridae, Amphizoidae, Hygrobiidae and Carabidae. Members of all currently recognized tribes of Dytiscidae are represented except Anisomeriini Brinck, Hydronebriini Guignot and Carabhydrini Watts. Emphasis is placed on identifying informative characters from the female genital system that comprise 34 of the resulting 101 total characters. The consensus of the most parsimonious trees is well resolved and supports recognition of ten subfamilies of Dytiscidae including; Matinae van den Branden, Laccophilinae Gistel, Coptotominae van den Branden, Copelatinae Erichson, Hydroporinae Aubé, Agabinae Thomson, Colymbetinae Erichson, Lancetinae van den Branden and Dytiscinae Leach. Also, Hydrodytes Miller, NEW GENUS, is erected and placed in its own subfamily, Hydrodytinae, NEW SUBFAMILY, to include two species previously placed in Agaporomorphus Zimmermann (Copelatinae), H. opalinus (Zimmermann) (NEW COMBINATION) and H. dodgei (Young) (NEW COMBINATION). Hydrodytinae is sister group to Hydroporinae and is diagnosed by the presence of anterior apodemes on the gonocoxae, several characters of the metendosternite (each synapomorphic with Hydroporinae), lack of pseudotetramerous pro- and mesotarsi, lack of a declivitous prosternum and prosternal process, the scutellum visible with the elytra closed (all plesiomorphic), the rami of the female genitalia sinuate and dorsally with an opalescent sheen (each autapomorphic for Hydrodytinae). Matinae is resolved as the sister group to the remaining Dytiscidae. Hyphydrini Sharp is found to be paraphyletic with respect to Pachydrini Biström, Nilsson and Wewalka, and the latter is relegated to a junior subjective synonym of the former (NEW SYNONYMY). Hydroporini Aubé and Hygrotini Portevin are found to be para- or polyphyletic. No changes are made to the classification of these taxa since character evidence is relatively weak, and taxon sampling within Hydroporinae is inadequate to justify changes. Carabdytes Balke, Hendrich and Wewalka is found to be the sister taxon to the remaining Colymbetinae, and because of its unique combination of characters and phylogenetic placement it is included in its own tribe, Carabdytini Pederzani (RESURRECTED). All other examined tribes of Dytiscidae are monophyletic. The female genitalia are described and figured for numerous taxa across the family, and numerous other characters are described and figured. The evolution of various features of the female genitalia within Dytiscidae is discussed. The resulting phylogenetic hypothesis is compared and contrasted with other phylogenetic proposals.