Spermatogenesis is an intensively studied developmental process in Drosophila melanogaster and Drosophila hydei. Nonetheless, some phases of the development of the germ cells are still hardly understood. The main gap of information is with respect to the germ line-soma differentiation and the formation of the gonad. Our knowledge about pole cell formation and its migration to the testis anlagen is still incomplete, and nearly nothing is known about the genetic control of these processes. In addition, some of these processes may be shared by both spermatogenesis and oogenesis. Therefore, this chapter will deal mainly with later stages of spermatogenesis, i.e., with the genetic control of the spermatogenesis sensu strictu.