Bariatric surgery is the most durable intervention for severe obesity. Appropriate candidates for surgery include those with a body mass index over 40 kg/m(2), or those with a BMI over 35 kg/m(2) who also have weight-related comorbidities. Bariatric procedures are categorized as restrictive, where food intake is limited by a small gastric 'pouch'; malabsorptive, where the length of intestine available for nutrient absorption is decreased; or a combination of both. Although pure malabsorptive procedures, such as the now-historical jejunoileal bypass, achieve greater weight loss than restrictive procedures, they are generally associated with more postoperative metabolic problems. The Roux-en-Y gastric bypass is currently considered the gold standard bariatric procedure for most patients. It results in excellent weight loss with minimal complications, but does require life-long vitamin supplementation. Compliance with vitamins and supplements is also mandatory after malabsorptive procedures. With these procedures, decreased oral intake, as well as altered absorption of nutrients from the GI tract, results in potentially low blood levels of a variety of micronutrients, especially iron, vitamin B12 and folate. Bariatric surgery also improves the comorbid conditions that are associated with obesity, such as diabetes, hypertension, dyslipidemia, obstructive sleep apnea, obesity hypoventilation, gastroesophageal reflux disease, asthma, venous stasis, polycystic ovary syndrome and pseudotumor cerebri. The resolution of diabetes is secondary to weight loss and may also be due to alteration of the enteroinsular axis.