Obesity has gained prominence as a main cause of preventable illness and death in the developed world. Surgical therapy for obesity is extremely effective in terms of weight reduction and amelioration of comorbidities. Bariatric procedures are not simply cosmetic operations, however, and involve considerable manipulation of the gastrointestinal tract to induce weight loss. The metabolic consequences of these procedures can be severe if not preempted with relatively simple postoperative precautions on the part of the patient and surgeon. Modern bariatric procedures are much safer than their predecessors, but nutritional and metabolic changes must be anticipated and compensated to fully realize the benefits of surgery. The metabolic consequences of the now outdated jejunoileal bypass, and the more modern Roux-Y gastric bypass, gastroplasty, and biliopancreatic diversion, are presented here, along with specific considerations of patient populations.