Congenital hyperinsulinism (CHI) is a heterogeneous disease with variable onset, non- or hypoketotic hypoglycaemia, onset from birth to adulthood and a persistent, intermittent, or transient course with possible later conversion to non-autoimmune diabetes. Giving insights to beta cell function, CHI mutations are now known in eight genes (ABCC8, KCNJ11, GLUD1, GCK, HADH, SLC16A1, HNF4A and UCP2). However, 40-50% of the patients are still genetically unexplained. CHI can be dominantly or recessively inherited or may occur de novo. A number of syndromes can be associated with CHI.