Male sexual differentiation involves a cascade of events initiated by the presence on the Y chromosome of the SRY gene, which causes the indifferent gonad to develop into a testis. Hormonal products of the testis, predominantly testosterone and Mu¨llerian inhibiting substance (MIS), then control the sexual differentiation of the developing foetus. SRY is a transcription factor; however, target genes for its action have yet to be identified, because the DNA recognition sequence for SRY is found in many genes. Therefore the study of intersex disorders is being used to identify other genes active in the pathway of sexual differentiation. Genes identified as being important in the differentiation of the indifferent gonad include WT1 (abnormal in Denys Drash syndrome) and SF‐1. The DSS locus may contain a gene that controls ovarian differentiation, and SOX9 (identified from campomelic dysplasia) is required for testis differentiation. In addition to playing a role in the development of the bipotential gonad, SF‐1 may also activate MIS gene expression in the testis, causing regression of Mu¨llerian structures. Luteinizing hormone and its receptor are required for Leydig cell differentiation, and the testosterone biosynthetic enzymes (P450scc, 3β‐hydroxysteroid dehydrogenase, P45017α and 17β‐hydroxysteroid dehydrogenase) are all necessary for masculinization of external genitalia. 5α‐Reductase is required for the production of dihydrotestosterone, and the androgen receptor mediates the action of both testosterone and dihydrotestosterone. The identification of abnormal genes in other disorders of sexual differentiation is likely to provide further information about the factors required for testicular development and function.