Hypoglycaemia is one of the most common biochemical abnormalities observed in the neonatal, infancy and childhood periods. Despite the commonality there is still confusion about the definition and management of hypoglycaemia. Hypoglycaemia can be due to many causes (including endocrine and metabolic) in the neonatal, infancy and childhood period. For example hyperinsulinaemic hypoglycaemia is the most severe form of hypoglycaemia in the neonatal period whereas ketotic hypoglycaemia presents in the childhood only during an intercurrent illness. Thus having an understanding of normal glucose physiology will not only help the clinician to understand the biochemical basis of hypoglycaemia but will also allow the clinician to organise appropriate investigations and institute the correct management. The early recognition and prompt management of hypoglycaemia is the cornerstone in preventing brain injury. During this talk I will give an overview of glucose physiology, review the causes of hypoglycaemia in the neonatal and childhood periods and discuss the different management approaches.