Hypoglycaemia is the most frequent and serious complication of insulin therapy and is three times more common in those who are intensively treated. Impaired awareness of hypoglycaemia is a major limitation to achieving tight diabetes. Asymptomatic biochemical hypoglycaemia occurs more frequently during routine blood glucose monitoring and this should alert the clinician that the individual is developing hypoglycaemic unawareness. Though the etiology of hypoglycaemic unawareness is multifactorial, possible mechanisms include chronic exposure to low blood glucose, antecedent hypoglycaemia, recurrent severe hypoglycaemia and the failure of counter-regulatory hormones. Diabetic patients with history of impaired awareness of hypoglycaemia perform poorly on cognitive function testing and this may be restored by relaxation of glycaemic control. The question of whether human insulin produces hypoglycaemia awareness has been hotly debated but this does not appear to be a major problem considering the number of patients on human insulin. The decision to use one or other of the insulins therefore rests entirely with the physician. Low blood glucose awareness training programmes can help to identify and prevent hypoglycaemia unawareness. Impaired awareness of hypoglycaemia can probably be reversed by scrupulous avoidance of hypoglycaemia.