Bariatric surgery is currently the most effective and durable treatment option for extreme obesity. Restrictive procedures, such as AGB and SG, limit gastric capacity and, thus, food intake while leaving the gastrointestinal tract intact. Malabsorptive procedures, such as BPD, shorten the length of the intestine to decrease nutrient absorption. Combined procedures, such as RYGB, include restriction and gastrointestinal rearrangement. Procedures that bypass segments of the gut are associated with greater weight loss and greater improvements in comorbid conditions than is gastric banding. This may be due, in part, to the differential effects of gastrointestinal rearrangement on the secretion of orexigenic and anorexigenic gut peptides that regulate appetite, glucose homeostasis, and body weight. Bariatric surgery is generally associated with low rates of perioperative and postoperative morbidity and mortality, although rigorous comparative safety data are lacking. High-quality, long-term, randomized, controlled trials are needed to compare the efficacy, safety, and cost effectiveness of the various bariatric surgery procedures with each other, as well as with intensive nonsurgical weight loss interventions.