The general morphology, reproduction and development of the family Onuphidae are reviewed and supplemented with new observations, emphasizing ontogenetic changes. The following features were found to be associated with the juvenile state: distinctive colour pattern, ceratophores of antennae with few rings, absence of frontal palps and tentacular cirri, presence of eyes, smaller number of modified setigers, lesser developed branchiae, early occurrence of sub acicular hooks, presence of compound falcigers in anterior and far posterior setigers. Five new genera are described, bringing the number of recognized genera in the family to 22. A key, diagnoses and illustrated definitions to all genera are given. The relationships within the family are analyzed using morphological, ecological and life history characters. Two subfamilies: Hyalinoeciinae, n. subf., and Onuphinae are erected. The two subfamilies differ in the presence or absence of notosetae, position of sub acicular hooks and lower limbate setae, number of anal cirri, primary envelope of oocytes and arrangement of their nurse cells. Two groups of genera are recognized in each subfamily. The Hyalinoeciinae includes the Nothria group (consisting of Nothria and Anchinothria, n. gen.) and the Hyalinoecia group (consisting of Hyalinoecia; Leptoecia; Neonuphis; HyalospiniJera). The Onuphinae includes the Diopatra group (consisting of Notonuphis; Paradiopatra; Diopatra; Epidiopatra; Brevibrachium, n. gen.; Longibrachium, n. gen.; Rhamphobrachium; Americonuphis) and the Onuphis group (consisting of Australonuphis; Hartmanonuphis, n. gen.; Hirsutonuphis, n. gen.; Aponuphis; Kinbergonuphis; Mooreonuphis; Onuphis; Heptaceras). It is hypothesized that the Onuphidae have a southern centre of origin and radiated from epifaunal habitats to world-wide distributions from the shallowest to the deepest depths.