Formerly known as 'intersex' conditions, disorders of sex development (DSDs) are congenital conditions in which chromosomal, gonadal or anatomical sex is atypical. A complete revision of the nomenclature and classification of DSDs has been undertaken, which emphasizes the genetic aetiology of these disorders and discards pejorative terms. Uptake of the new terminology is widespread. DSDs affecting gonadal development are perhaps the least well understood. Unravelling the molecular mechanisms underlying gonadal development has revealed new causes of DSDs, although a specific molecular diagnosis is made in only ∼20% of patients. Conversely, identification of the molecular causes of DSDs has provided insight into the mechanisms of gonadal development. Studies of N-ethyl-N-nitrosourea mutagenesis in the mouse, and multigene diagnostic screening and genome-wide approaches, such as array-comparative genomic hybridization and next-generation sequencing, in patients with DSDs are accelerating the discovery of genes involved in gonadal development and DSDs. Furthermore, long-range gene regulatory mutations and multiple gene mutations are emerging as new causes of DSDs. Patients with DSDs, their parents and medical staff are confronted with challenging decisions regarding gender assignment, genital surgery and lifelong care. These advances are refining prognostic prediction and systematically improving the diagnosis and long-term management of children with DSDs.