Congenital hyperinsulinism is characterized by the unregulated secretion of insulin from pancreatic Beta-cells. The inappropriate insulin secretion causes severe and persistent hypoglycaemia, which is a potent cause of brain damage if inappropriately managed. So far mutations in 5 different genes have been described which lead to inappropriate insulin secretion. The most common cause of congenital hyperinsulinism is autosomal recessive mutations in the genes ABCC8 and KCNJ11 encoding the 2 subunits (SUR 1 and Kir6.2, respectively) of the pancreatic Beta-cell ATP-sensitive potassium channel. Autosomal dominant mutations in the genes encoding glucokinase (GCK) and glutamate dehydrogenase (GLUD1) lead to inappropriate insulin secretion by increasing the ATP/ADP ratio in the Beta-cells. Autosomal recessive mutations in the HADHSC gene (encoding the enzyme short-chain L-3-hydroxyacyl-CoA dehydrogenase) have been linked to defects in fatty acid oxidation and hyperinsulinism. Finally some patients have been described with exerciseinduced hyperinsulinaemic hypoglycaemia but the genetic basis of this is unclear at present. Recent advances in 18fluoro-L-Dopa positron emission tomography scanning suggest that this is a highly sensitive method for differentiating diffuse from focal disease as well as accurately locating the focal lesion. Despite huge advances in the last 10 years the mechanisms leading to hyperinsulinaemic hypoglycaemia are still unknown in >50% of patients.