Insulin-dependent diabetes mellitus is a polygenic disease with an environmental component. Technological advances and large collection families allowed genetic factors understanding. On clinical practice, two questions could be raised. First, will the genetic markers be of interest in disease prediction either in families studies or in the general population? Second, will the genetic approach explain the physiopathological process of the disease? Initially, the gene candidate approach led to the identification of two important loci. Linkage with the HLA locus showed the importance of the autoimmune part. Linkage of insulin-dependent diabetes mellitus with insulin locus gave a mechanistic answer for disease susceptibility. These two loci can be used as prediction markers, but only in family studies. Since 1993, a whole genome approach was performed and led to the identification of other susceptibility loci. These initial results are in progress and should have important implications for public health strategies.