The symptoms of hypoglycaemia are fundamental to the early detection and treatment of this side-effect of insulin and oral hypoglycaemic therapy in people with diabetes. The physiology of normal responses to hypoglycaemia is described and the importance of symptoms of hypoglycaemia is discussed in relation to the treatment of diabetes. The symptoms of hypoglycaemia are described in detail. The classification of symptoms is considered and the usefulness of autonomic and neuroglycopenic symptoms for detecting hypoglycaemia is discussed. The many external and internal factors involved in the perception of symptoms are reviewed, and symptoms of hypoglycaemia experienced by people with Type 2 diabetes are addressed. Age-specific differences in the symptoms of hypoglycaemia have been identified, and are important for clinical and research practice, particularly with respect to the development of acquired hypoglycaemia syndromes in people with Type 1 diabetes that can result in impaired awareness of hypoglycaemia. In addition, the routine assessment of hypoglycaemia symptoms in the diabetic clinic is emphasized as an important part of the regular review of people with diabetes who are treated with insulin.