In the last five years, our knowledge about the heterogenous syndrome of congenital hyperinsulinism (HI) has expanded explosively. HI may be familiar or sporadic, mild or severe, transitory or persistent, and histologically focal or diffuse. At least 63 disease-causing mutations have been found in the genes for the beta cell's ATP-dependent potassium channel, whose elements are the sulphonylurea receptor, SUR1, and Kir6.2. Other mutations cause enhancement of the glucose-stimulated ATP production in the beta cell. The resulting non-functional, or closed, potassium channel causes hypersecretion of insulin. Genetic screening has succeeded in detecting mutations in less than 50% of HI-patients. Genotype-phenotype relations, diagnosis and treatment are reviewed.