Since the discovery of SRY as the main testis-determining gene more than 25 years ago, at least 30 other single gene conditions have been identified that can result in differences/disorders in sex development (DSD). Some of these are associated with classic steroidogenic blocks or related conditions such as CAH, whereas others are associated with alterations in gonad determination and development. Reaching a specific genetic diagnosis can have implications for understanding the condition, the need for monitoring associated features, defining tumour risk and fertility options, as well as counselling an individual and their family about inheritance patterns. A genetic approach may be the only way to reach a specific diagnosis, especially when there are no pathognomonic biochemical tests available, or in the adult clinic when gonadectomy may have been performed or when information about historic investigations may be limited. In this session, I will review the mechanisms and pathways underlying sex development in humans; provide an overview of the classification of DSD; highlight some recent advances in the field (e.g. NR5A1/SF-1, DHX37) and discuss the relative contribution of different causes in the paediatric and adult clinic. We will consider some circumstances where genetic diagnosis is important, and also how new technologies such as next generation sequencing and single-cell RNASeq can help us to understand mechanisms and reach a genetic diagnosis more quickly.