The male and female reproductive systems in three typical species of Nematocera, order Diptera, representing the families of Chironomidae, Anisopidae and Mycetophilidae, have been studied and their development followed through the larva and pupa. A description has been given of the parts in each case in the imaginal fly, and as seen in development and a nomenclature of parts adopted in accordance with the homologies ascertained. In each case the basal plan is similar, but there is wide departure in the functional adaptations and form of the parts, so that without a study of the development the nature of some of the parts shown to be developmentally homologous would not be apparent. Briefly the reproductive system comprises in both sexes: (a) A primary reproductive system consisting of a pair of gonads in the sixth segment with mesodermal strands forming the primative mesodermal oviducts and vasa efferentia and linking the gonads with the ectodermal efferent ducts. (b) A secondary reproductive system of ectodermal origin which forms the greater part of both male and female efferent systems and which originates from hypodermal proliferative rudiments on the hypopygial segments and which may in various degrees replace by overgrowth the mesodermal ducts. (c) Appendicular parts forming in the male clasping organs (gonocoxites) and gonapophysial organs (paramere lobes) and also intersegmental inflexions and sclerotizations such as the atrium and vaginal apodeme in the female and the theca, penis sheath and, when present, an eversible sac or endotheca in the male. In the male recognizable homologous processes are the consecutive invaginations of the twin ejaculatory ducts and the median invagination of the penis tube, which later may be followed by a third intersegmental inflexion, that of the genital pouch excavating the penis and in Anisopus forming in its walls sclerotizations of the pompetta and piston apodeme. In the female are three recognizable invaginations, that taking origin from the eighth sternite rudiments, which conjointly by their peripodial cavity form the vagina with the spermathecal and later the common oviduct invaginations, and that from the conjoined hypodermal buds on the ninth sternite forming the caecus (the accessory glands).