Genes containing the DM domain DNA-binding motif regulate sex determination and sexual differentiation in a broad variety of metazoans, including nematodes, insects, and vertebrates. They can function in primary sex determination or downstream in sexual differentiation, and they can act either throughout the body or in highly restricted cell types. In vertebrates, several DM domain genes- DMRT genes-play critical roles in gonadal differentiation or gametogenesis. DMRT1 has the most prominent role and likely regulates testicular differentiation in all vertebrates. In the mammalian gonad, DMRT1 exerts both intrinsic and extrinsic control of gametogenesis; it is required for germ cell differentiation in males and regulates meiosis in both sexes, and it is required in supporting cells for the establishment and maintenance of male fate in the testis. These varied functions of DMRT1 serve to coordinate gonadal development and function. In other vertebrates, DMRT1 regulates gonadal differentiation, and it also appears to have played a central role in the evolution of new sex-determining mechanisms in at least three vertebrate clades. This chapter focuses on the regulation of vertebrate gametogenesis by DMRT1.