Genetic factors are essential to the occurrence of insulin-dependent diabetes (IDD) and non-insulin-dependent diabetes (NIDD), and all that environmental factors do is facilitate the development of diabetes in genetically predisposed subjects. Recent advances in molecular biology have improved our understanding of diabetic heredity. IDD is closely linked to the HLA region of chromosome 6. Ninety percent of IDD belong to the DR3 or DR4 group. The occurrence of IDD is facilitated by a peculiar conformation of the HLA DQ molecule which permits the presentation of antigens to the T-cells. Other genes still have to be discovered since IDD seems to be of polygenic origin. NIDD is even more "hereditary" than IDD, but owing to the lack of an unquestionable marker the responsible genes cannot be located with certainty. Several possible genes such as those of insulin, insulin receptor and glucose transporter, are suspected, at least in some forms of NIDD--a clinically and biologically highly heterogeneous disease. Widespread family studies should, in a not too distant future, locate the responsible gene thereby leading to early detection or even prevention of NIDD.