Hypoglycemia (from ancient Greek ὑπογλυκαιμία, hypo-, glykys, -haima) means low blood sugar. It is a common clinical problem in neonates, less common in infants, and rare in older children. It can be caused by various conditions. The most common cause of severe hypoglycemia in childhood is congenital hyperinsulinism. Symptoms of hypoglycemia depend on the age of the child, the rate at which it develops, the cause or the accompanying symptoms of the disease that causes the hypoglycemia and the individual susceptibility to hypoglycemia. Infants and children with asymptomatic or symptomatic hypoglycemia have been shown to have neurocognitive defects at the time of hypoglycemia. The clinical history, examination and some simple clinical and biological tests may allow diagnosing the vast majority of patients presenting with hypoglycaemia. The ability to properly sort through the differential diagnosis of hypoglycemia depends on obtaining a blood sample at the time of hypoglycemia. This sample is used to measure the various metabolic precursors and hormones involved in glucose regulation, including glucose, insulin, growth hormone, cortisol, lactate, pyruvate, beta-hydroxybutyrate, free fatty acid, carnitine and C-peptide. Short-term treatment of hypoglycemia consists of an intravenous bolus of glucose 10% 2.5 mL/kg. After the bolus is administered, an IV infusion should be continued. This should be adjusted to maintain the plasma glucose level at more than 3 mmol/L.