The sex determining gene, Sry, determines the sex of the organism by initiating development of a testis rather than an ovary from the cells of the bipotential gonad. In the 10 years since the discovery of Sry, new genes and cellular pathways that operate in the establishment of the gonadal primordium and the initiation of testis development have been discovered. Experiments defining mechanisms downstream of Sry are providing clear examples of how a regulatory transcription factor initiates cellular processes including proliferation and cell migration, which in turn influence architectural patterning, fate commitment, and differentiation of cells within an organ.