Monogenic diabetes mellitus is a type of diabetes, where genetics without any other factors is strong enough to cause the disease. According to the clinical features monogenic diabetes can be divided to the mild familial early onset diabetes, familial fasting hyperglycemia, diabetes with extrapancreatic features and neonatal diabetes mellitus. During the last several years the number of genes causing monogenic diabetes has continuously increased. The clinical picture of the monogenic diabetes is very heterogeneous, thus DNA analysis is required for identification of the diabetes etiology, which influences also the choice of treatment. This article is an overview of current knowledge on monogenic diabetes, focusing at the clinically and epidemiologically most important forms.