True porpoises are a morphologically distinctive and evolutionarily old group of odontocete cetaceans classified as the family Phocoenidae. They are distinct from members of the family Delphinidae, with which they have sometimes been classified. Re-examination of all living and fossil species of phocoenids yields new information on the evolution of the family and indicates the need for significant taxonomic changes. Two separate lines of descent within the group are classified as the subfamilies Phocoeninae and Phocoenoidinae. The living southern hemisphere spectacled porpoise, Phocoena dioptrica, is actually more closely related to the North Pacific Dali's porpoise, Phocoenoides dalli, than to other species of Phocoena. It therefore belongs in the subfamily Phocoenoidinae with Phocoenoides but represents a separate genus, Australophocaena (new genus). The earliest unquestioned fossil phocoenids have been found only in rocks around the Pacific basin. They include latest Miocene and Pliocene species of Piscolithax and the Late Miocene Salumiphocaena stocktoni (new genus) in the Phocoenoidinae, and an undescribed latest Miocene species in the Phocoeninae. World climate has influenced past and present distributions of phocoenids, and Australopbocaena dioptrica is possibly the antitropical counterpart of Phocoenoides dalli. All species of living phocoenids show similarities caused by convergent evolution, resulting in part from the phenomenon of paedomorphosis.