Genetic factors play an important role in various forms of diabetes mellitus (DM), but inheritance is complex and interacts with environmental factors. Although in most cases type 2 DM (T2DM) and T1DM are polygenic disorders, several monogenic forms have been identified. Among them, maturity‐onset diabetes of the young (MODY) has been the most intensively investigated. MODY is a group of six different forms of monogenic diabetes, characterized by insulin secretion defects in pancreatic β‐cells, supposed to be responsible for 2–5% of all cases of diabetes. The most common are MODY2 and MODY3, caused by mutations in the genes encoding glucokinase and hepatocyte nuclear factor 1‐alpha respectively. MODY2 is characterized by glucose sensing defects, leading to an increase in insulin secretion threshold. This causes lifelong sustained and mild hyperglycaemia from birth, most often in non‐diabetic levels. Diagnosis is incidental in most cases. These patients are asymptomatic, seldom need treatment and rarely present chronic complications. MODY3 is characterized by a severe insulin secretion defect in response to glucose. Diagnosis is made usually in adolescence and early adulthood, often by osmotic symptoms. Hyperglycaemia is progressive, and patients frequently need treatment with oral drugs or insulin some time in their follow up. This group seems to have a marked sensitivity to sulphonylureas compared to other types of diabetes. The recognition of MODY as a monogenic disorder and a thorough understanding of its pathophysiology are important for correct diagnosis and treatment, with great impact on prognosis. Besides, the study of these forms of diabetes brings important contributions to the understanding of glucose homeostasis as a whole.