Hypoglycaemia is one of the most common acute complications of type 1 diabetes mellitus. Regardless of the patients’ age or mode of treatment (intensive insulin treatment with insulin pens or sensor-augmented continuous subcutaneous insulin infusion), it represents a major challenge in their daily lives. The diabetology team has to be aware of the numerous reasons for hypoglycaemia and consider them when establishing the differential diagnosis of a patient. The mechanism of hypoglycaemia is multi-faceted and is influenced by various factors that are unique to each and every patient. It is important to explore the reasons for episodes of hypoglycaemia occurring in each individual patient. This is the only way to enable the patients to resolve the problem of hypoglycaemia with their internal resources and professional help. The patients can thus enjoy a good quality of life, rather than be endangered by hypoglycaemia. In this paper, we have summarized the recent knowledge in this field, which is essential for anyone professionally dealing with hypoglycaemia.