The molecular mechanisms of gonadal development are a complex process, which involves the tightly regulated differentiation of a bipotential embryonic gonad into either testes or ovary. Once this has occurred, the phenotypic and gonadal sex of an individual has been genetically determined. This process, however, may not always be so straightforward. By studying individuals who are sex reversed or who have ambiguous genitalia, the discovery of the handful of sex-determining genes that we know of today was made possible. It was not until recently that the transcription factors SRY, DAX1, SOX9, SF-1, and WT1 were recognized to be involved in gonadal development. Dissecting the molecular pathway of mammalian sex determination will be crucial in understanding the development of the gonads and the pathophysiology of human disorders of sex determination.