Hypoglycaemia (blood glucose concentration below the normal range) is a common and feared complication of exogenous insulin and insulin secretagogue treatments for diabetes. Severe hypoglycaemia is defined as an episode in which third-party intervention is required because of cognitive impairment secondary to the glucose lack. Even non-severe episodes can be unpleasant, as the body mounts a symptomatic stress response to the glucose fall. These protective, counterregulatory responses are impaired in diabetes, increasingly so with increasing diabetes duration, increasing the risk of severe hypoglycaemia. Much can be done to reduce individual risk of severe hypoglycaemia, starting with adjustment of medications and, for insulin users, structured education in how to use insulin flexibly; through use of technologies such as analogue insulins, insulin infusion, and continuous glucose monitoring, with β cell replacement a final step. Technological solutions should be accompanied by support for psychological issues of having diabetes and hypoglycaemia, which are challenging for patients and families. This chapter discusses the epidemiology of hypoglycaemia in diabetes, explores the pathogenesis of impaired awareness and severe hypoglycaemia and discusses the treatment pathway.