Diabetes mellitus is a chronic, lifelong condition which can affect people of all ages, and is increasing in prevalence. Hypoglycaemia is probably the most common acute problem suffered by patients with diabetes. It is also a serious medical emergency with potentially fatal outcomes, and is the most common reason for patients with diabetes attending an accident and emergency (A&E) department. It is also a major source of anxiety for diabetics, particularly those controlled on insulin, and unfortunately, in the move towards ever tighter glycaemic control, it is inevitable that diabetics will continue to suffer from hypoglycaemia. This article examines the pathophysiology of hypoglycaemia and some of its main causes, and will look at the clinical management of the patient with hypoglycaemia, both in the community and in the A&E department. The importance of the recognition and prompt treatment of hypoglycaemia, and of the investigation of hypoglycaemia with no obvious cause are also discussed. Part 2 of the series will explore the pathophysiology and clinical management of diabetic emergencies involving hyperglycaemia, including both diabetic ketoacidosis and the rarer hypernatremic, non-ketotic coma.