The gastroenterologist, whether in academic or clinical practice, must face the reality that an increasingly large percentage of adult patients are morbidly obese. Morbid obesity is associated with significant morbidity and mortality including enhanced morbidity from cardiovascular, cerebrovascular, hepatobiliary and colonic diseases. Most of these associated diseases are actually preventable. Based on the 1991 NIH consensus conference criteria, for most patients with a body mass index (BMI=weight in kilograms divided by the height in meters squared) of 40 or more, or for patients with a BMI of 35 or more and significant health complications, surgery may be the only reliable option. Currently in the United States, over 250,000 bariatric surgical procedures are being performed annually. The practicing gastroenterologist in every community, large and small, must be familiar with the various surgical procedures together with their associated anatomic changes. These changes may dramatically increase the prevalence of nutritional deficiencies and profoundly alter the clinical and endoscopic approaches to diagnosis and management.