The hypoglycemia hyperinsulinemic of the infancy (HHI) is an emergency in the neonatal period. After a short period of fast the avid brain runs out of its main energy substrate. The authors overhaul the diagnosis of HH, not only in the neonatal period, but also in the late infant and in the adolescence. The aspects of the molecular alterations found in these cases, as well like the description of the main mutations are also approached.