The pathways that control insulin secretion and regulate pancreatic β-cell mass are crucial in the development of diabetes mellitus. Maturity-onset diabetes of the young comprises a number of single-gene disorders affecting pancreatic β-cell function, and the consequences of mutations in these genes are so serious that diabetes develops in childhood or adolescence. A genetic basis for the more common form of type 2 diabetes, which affects 10–20% of adults in many developed countries, is less clear cut. It is also characterized by abnormal β-cell function, but other tissues are involved as well. However, in both forms identification of causative and susceptibility genes are providing new insight into the control of insulin action and secretion, as well as suggesting new treatments for diabetes.