Maturity-onset diabetes of the young (MODY) is a dominantly inherited form of non-ketotic diabetes. It usually develops in childhood, adolescence or young adulthood. The disease is heterogenous regarding the genetic and clinical features. Until now, 6 causal genes have been identified. According to clinical evidence there is at least one further MODY gene. MODY2 is caused by mutations in the glucokinase gene, which encodes the signal protein for insulin secretion. The remaining MODY subtypes are the result of mutations in genes coding for transcription factors. MODY2 and MODY3 are by far the most frequent forms. Patients with MODY2 have mild, asymptomatic and stable hyperglycemia that is present from birth. They rarely develop microvascular disease, and seldom require pharmacologic treatment. MODY3 patients usually develop severe hyperglycemia after puberty, which often leads to the diagnosis of diabetes type 1. Despite the progression of hyperglycemia, sensitivity to sulfonylureas is retained in MODY3 patients for many years. Diabetic retinopathy and nephropathy frequently occur in MODY3 patients. Other risk factors are not present. The frequency of cardiovascular disease is not increased. Due to the pleiotropic character of the transcription factors most MODY subtypes are diseases with multi-organ involvement in addition to diabetes. MODY5 appears to be much more frequent than originally assumed. This form is associated with pancreatic atrophy, renal morphologic and functional abnormalities, genital tract malformations and pathological liver test. Compared to MODY2, 3, and 5, the remaining subtypes of MODY have a much lower prevalence. Molecular diagnosis has important consequences for prognosis, family screening and therapy.