Abstract : Obesity, whose prevalence is increasing in industrialized countries, is recognized as a major risk factor for the development of type 2 diabetes. In contrast, the role of excess adiposity in the natural history of type 1 diabetes (T1DM) and its impact on therapeutic approaches are by far less known and most probably largely underestimated. The following items will be most particularly considered : 1) the accelerator role of obesity in the development of T1DM in young predisposed individuals; 2) the difficult diagnosis in presence of hybrid or double diabetes combining insulin secretory defect and insulin resistance; 3) the problem of managing both body weight and glucose control, especially in young females and adolescents; 4) the possible deleterious effects of intensive insulin therapy on body weight and some markers of the metabolic syndrome; 5) the worse cardiovascular prognosis of T1DM patients when obesity is present; 6) the possible role of other medications as adjunct therapy to insulin in insulin resistant obese T1DM patients; and finally 7) the expected benefit of bariatric surgery in some well selected T1DM patients with severe obesity.