Obesity and diabetes are rapidly reaching epidemic proportions in many parts of world and are becoming one of the major public health problems. Many studies have been performed to develop treatments for obesity and diabetes. In clinical aspect, for example, vitamin D was assumed to be a causal factor of obesity and diabetes, and the effect of vitamin D supplementation on the patients was assessed. In addition to clinical study, basic researchers have tried to elucidate the mechanisms of obesity and diabetes. Recent studies show novel techniques for finding etiologic factors in obesity and diabetes. These effort will accelerate progress toward total eradication of obesity and diabetes.