Hypoglycaemia is a serious, frequent and recurrent complication of treatment of diabetes mellitus which may become a direct danger to the patient's life. In clinical practice, it is best defined descriptively as Whipple's triad involving the presence of typical clinical symptoms of hypoglycaemia, low plasma glucose concentration and the disappearance of symptoms of hypoglycaemia after the administration of glucose. The overview article focuses, in more detail, on the issues of definition of hypoglycaemia based on the optimal threshold value ofglycaemia, classification ofhypoglycaemia based on its causes, rate of occurrence and factors provoking hypoglycaemia in patients with type 1 and 2 diabetes mellitus, as well as modern diagnosis and treatment of hypoglycaemic states.