This volume reviews our current understanding for how sex determination is initiated and how it results in sexual dimorphic development. Chapters discussing work on different model systems provide a basis for understanding similarities that exist between different species. Coverage includes discussion of sexual development of the soma in C. elegans; sexual development of the germline in C. elegans; sexual development of the soma in Drosophila; sexual development of the germline in Drosophila; sexual development of the soma in the mouse; sexual development of the germline in the mouse; control of sex-specific behavior in Drosophila; and control of sex-specific behavior in vertebrates.