Practically all types of diabetes mellitus (DM) result from complex interactions of genetic and environmental factors. Multifactorial and polygenic Type 1 DM is strongly influenced by genes controlling the immune system, mainly HLA-DQ and DR. In addition to this, many other predisposition loci, interacting with each other, have some influence on susceptibility to DM. Heterogeneous Type 2 DM, accounting for about 85% of all diabetic patients, is supposed to be induced by multiple genes defects involved in insulin action and/or insulin secretion. Other genetically influenced traits like obesity and hyperlipidemia are strongly associated with the Type 2. The group called Other specific types of DM include monogenic forms MODY 1-5 and many various subtypes of the disease, where the specific gene mutations have been identified. Both genetic and intrauterine environmental influences are likely to contribute to the abnormalities defined as Gestational DM.