Hypoglycemia of infancy and childhood represents a treatable cause of mental retardation and seizures. Most neonates with hypoglycemia have transient disorders, but with persistent hypoglycemia one must consider hyperinsulinism, hypopituitarism, or hereditary hepatic enzyme deficiencies. Outside of the neonatal period, ketotic hypoglycemia is the most common cause of hypoglycemia in childhood. One cannot overemphasize the value of obtaining certain diagnostic tests at the presentation of spontaneous hypoglycemia, including blood for insulin, cortisol, growth hormone, and urine for ketone bodies. Supportive treatment with intravenous glucose to maintain the blood glucose greater than 50 mg/dl is important until a diagnosis is established allowing specific therapy aimed at the underlying disorder.