The greatest achievement in the treatment of obesity and diabetes would be the development of bariatric/metabolic surgery. At the beginning, bariatric surgeries were developed to simply reduce body weight in morbidly obese subjects. Before long, it was discovered that diabetes and other metabolic complications of obesity could be placed in remission. The remission rate of diabetes after bariatric surgery is strikingly high and, in the case of Roux-en-Y gastric bypass surgery, diabetes remission commonly occurs immediately after the surgery, when significant weight loss does not take place. Therefore, the concept of bariatric surgery has evolved into metabolic surgery. Physiologic changes in gastrointestinal endocrine system following the anatomical changes made by bariatric/metabolic surgery are regarded as the major mechanisms of weight loss and diabetes remission. In this regard, the foregut and hindgut hypotheses were suggested as the mechanisms associated with diabetes remission. With the advent of sleeve gastrectomy, which does not bypass the foregut (duodenum and proximal jejunum) but increases the secretion of glucagon-like peptide-1, the foregut hypothesis is currently under attack. However, a single mechanism is not enough to explain the metabolic effect of bariatric/metabolic surgery. Further studies are warranted to elucidate the mechanisms of metabolic improvements after bariatric/metabolic surgery. (Korean J Med 2013;84:629-639)