In addition to achieving significant weight loss, bariatric/metabolic surgery causes improvement, and even complete remission, of type 2 diabetes mellitus (T2DM) in a substantial number of severely obese patients. Diabetic patients with body mass index (BMI) ≥35 kg/m2, especially those with other weight-related comorbidities and acceptable surgical risk and have not responded adequately to nonsurgical management, are candidates for metabolic surgery. Currently accepted metabolic operations include Roux-en-Y gastric bypass (RYGB), laparoscopic adjustable gastric banding (LAGB), sleeve gastrectomy (SG), biliopancreatic diversion (BPD), and the duodenal switch variant (BPD-DS). The operations differ in the degree of benefit they impart on an individual’s state of metabolic profile. The general understanding, based on the results of limited high-quality trials, suggests the presence of an antidiabetic efficacy gradient among standard operations (i.e., BPD > RYGB > SG > AGB). More extensive diversionary procedures are generally associated with greater weight loss and more profound metabolic benefits in the long term, but at the cost of more surgical complications. The choice of metabolic surgery requires precise assessment of risk versus benefit for each operation and must be individualized for each patient. Patient’s operative risk and severity of T2DM are among the main determinants in the choice of metabolic surgery. In appropriate-risk patients with prediabetes or established T2DM, RYGB is the best overall option. The recommendations of this chapter are subject to change as more comparative studies become available.