For the understanding and interpretation of hypoglycemia it is important to know the many complex endocrine and metabolic regulations in the homoeostasis of blood glucose. Glucose-absorption, distribution and availability, glycolysis, production and utilization of glycogen as well as gluconeogenesis are important steps of this homoeostasis, and hypoglycemia always reflects a disturbance in it. When blood glucose is low the availability of energy for the brain is decreased if no alternative energy sources like lactate or ketones are provided. Hypoglycemia is more often in the neonatal period than in later childhood. The causes can be divided into different groups according to pathogenetic mechanisms. Within each group again many singular defects are known. Fructose-1,6-diphosphatase deficiency, hereditary fructose intolerance, glycogenosis type I and so called "ketotic hypoglycemia" are given as examples to elucidate special clinical and biochemical aspects of inborn errors of carbohydrate metabolism.