Eleven families of the isopod suborder Valvifera are diagnosed, and a key to differentiate them is presented. Three families, Antarcturidae, Rectarcturidae, and Arcturididae, are erected as new on the basis of a cladistic analysis of 14 families, genera, and genus-groups. Phylogenetic relationships between the families are suggested. The genera of each family, 81 in all, are listed. The generic name Austridotea, previously a nomen nudum, is made available within the Idoteidae. Differentiation of the families is based on fusion of the head and pereionite 1, length of the peduncle of pleopod 1, the male first pleopods, and fusion of the penes. Three families (Chaetiliidae, Holognathidae, Idoteidae) have pereionite 1 free from the head; pereiopod 1 subchelate, similar to others or larger; all pleopods simple and similar; and penes free or a basally fused penial plate. Those families with pereionite 1 fused to the head, pereiopod 1 smaller than other pereiopods and with different structure, an elongate pleopod 1 peduncle, and a fused penial plate have either the exopod of the male pleopod 1 laminar and excavate laterally (Arcturidae) or the exopod thickened and diagonally grooved (Arcturididae, Antarcturidae, Austrarcturellidae, Holidoteidae, Pseudidotheidae, Rectarcturidae, Xenarcturidae).