Patients with type 2 diabetes mellitus experience a complete and comfirm diabetic remission after bariatric surgery. Although weight-loss, reduction of food intake and other factors may play important roles in diabetic resolution after bariatric surgery, the major mechanism is the change in gastrointestinal hormones. Further research is essential to better understand these mechanisms and bariatric surgery may ultimately become a major tool in the treatment of type 2 diabetes mellitus.