Glucose is derived from three sources: intestinal absorption, glycogenolysis, and gluconeogenesis. Hypoglycemia in child is often attributed to depletion of glycogen stores. However, recently, congenital hyperinsulinism becomes an important cause of hypoglycaemia in early infancy. Mutations in the genes encoding SUR1 and KIR6.2 are the most frequent genetic causes of hyperinsulinism followed by mutations in the glutamate dehydrogenase (GDH) gene which encodes hyperinsulinism/hyperammonaemia (HI/HA) syndrome.