Hypoglycaemia remains one of the most confused and confusing issues in paediatric medicine. Controversy surrounds the definition of hypoglycaemia, the mechanism of adverse effect on the brain, and the practical approach to monitoring, management and treatment. A personal conspectus is presented on why such controversies have arisen, before reviewing recent advances in understanding the pathophysiology of hypoglycaemia and its effects on the developing central nervous system. A practical approach to the diagnosis of the cause of hypoglycaemia is presented which depends crucially upon measuring metabolic and endocrine interrelationships in the same blood sample obtained at the time of hypoglycaemia.