The hypoglycaemic coma is a severe complication for type 1 diabetic patients. Rarely fatal it may be associated with various paroxysmal accidents, potentially harmful, especially during driving. Hypoglycaemia certainly alters the quality of life because it markedly increases the anxiety of both the patient and his/her family. It is considered as a major limiting factor in the glycaemic management of type 1 diabetic patients. Being the consequence of numerous causal factors, hypoglycaemic coma is not always easy to prevent and may occur as a paroxysmal phenomenon, sometimes without obvious contributing circumstances. After having defined the various hypoglycaemic thresholds, we will analyse the pathophysiology of insulin-induced hypoglycaemia and of its hormonal counterregulation, and we will describe the hypoglycaemia unawareness phenomenon. These elements should help to better understand why a hypoglycaemic coma may suddenly occur in a diabetic patient. Some advices will also be given to reduce the risk of such a paroxysmal complication in patients with type 1 diabetes.