The mammalian sex-determining gene, SRY, was identified by positional cloning approximately 10 years ago. Since its discovery, intense research into this gene has been directed on two main fronts: elucidation of its function in development of the testis and examination of its singular evolutionary history. The role or SRY as the testis-determining factor (TDF) places it at a crucial point in the highly conserved morphogenetic process of vertebrate gonadogenesis. None of the genes that directly activate SRY nor any of its immediate downstream targets have yet been positively identified. Several genes, however, such as SF1, DAX1, and SOX9, whose spatial and temporal expression profiles overlap with that of SRY, are strongly implicated as co-regulators of gonadogenesis. Molecular genetic manipulation of these genes in mice has shown that they are indispensable to sexual development. Remarkably, its key position in this cascade of gene action has not protected SRY from strong yet poorly understood selective forces that have caused it to evolve rapidly in mammals. The evolution of SRY has been characterized not only by rapid sequence divergence within mammals, but also by structural changes such as intron insertion, gene amplification, and deletion.