The reproduction and larval development of spioniform polychaetes are reviewed. Asexual reproduction is relatively rare, being reported for only eight species belonging to the genus Pygospio and some polydorids. Both architomy and paratomy are known, with the latter limited to small species of Pseudopolydora (sometimes referred to Polydorella) and one species of Polydora. Architomy is often the primary form of reproduction in Pygospio elegans and contributes to the maintenance of large populations. Three types of eggs (thin egg envelopes, thick egg envelopes, smooth or reticulated, and thick egg envelopes, honeycombed), two types of oogenesis (extraovarian and intraovarian), and two types of sperm (ect-aquasperm and introsperm) occur in spioniforms. Egg and sperm type are restricted to specific clades. Eggs with thickened egg envelopes appear to be limited to spioniforms, whereas the thin egg envelope found in some spionids occurs in other polychaete families, suggesting that thin egg envelopes are plesiomorphic for spionids. Spermatophores occur in the spionid subfamily Spioninae and are formed in the male nephridia. Spioniforms exhibit a diversity of reproductive and larval patterns including broadcast spawning, external egg masses, brooding in capsules in tubes of females and brooding on the bodies of females. Poecilogony is unusually common in the Spionidae. A phylogenetic analysis demonstrates that reproductive and larval characters, when used in combination with selected adult characters, provide a more complete database to evaluate systematic and phylogenetic relationships than only adult morphology. Preliminary results of parsimony suggest that the Spionidae are paraphyletic and that its definition and the status of related spioniform polychaetes needs to be reassessed with regard to family level classification.