With the increasing number of bariatric surgical procedures being performed, outcome assessment is of even greater importance. Few randomized, controlled prospective trials have compared bariatric surgery to nonsurgical weight-loss treatments, and the quality of current outcome data is suboptimal. However, the available evidence suggests that bariatric surgery, and particularly gastric bypass, is the most effective weight-loss treatment for people with extreme (class III) obesity. In addition to reduced energy intake and to a lesser extent malabsorption, numerous other potential mechanisms related to bariatric surgery may play a role in promoting weight loss and improving comorbidities. After bariatric surgery, clinical improvement or resolution has been reported in 64% to 100% of patients with diabetes mellitus, 62% to 69% of patients with hypertension, 85% of patients with obstructive sleep apnea, 60% to 100% of patients with dyslipidemia, and up to 90% of patients with nonalcoholic fatty liver disease. A wide range of other weight-related conditions also appear to improve, and limited data suggest that overall mortality may decrease in patients undergoing bariatric surgery. Although not conclusive, evidence from available studies indicates that bariatric surgery is cost-effective. Further research with improved methodology is needed to define the mechanisms of action of bariatric surgery; to document its effect on long-term weight loss, comorbid conditions, and overall mortality; and to determine its cost-effectiveness.