Diabetes is one of the most common diseases in the United States, with 1.5 million new cases reported in 2005 and a projected 48.3 million diabetics in the year 2050. Despite the disease's remarkably high prevalence and the substantial efforts focused on pharmacological treatment of the disease, current dietary, behavioral, and medical therapies have achieved only limited success. For over 2 decades, bariatric surgeons have recognized that gastric bypass surgery causes a remarkable, durable improvement in type 2 diabetes mellitus. Remarkably, this normalization of glucose metabolism occurs within several weeks of surgery, long before substantial weight loss takes place. As early as in 1987, data have been available demonstrating that 83% of patients with diabetes and 99% of those with impaired glucose tolerance become euglycemic after gastric bypass surgery. Despite the availability of this effective treatment for diabetes, the primary therapy for type 2 diabetes mellitus has remained medical. In this article, we review some of the hypotheses about how bariatric operations affect glucose metabolism and diabetes as well as the research that has been done to elucidate these mechanisms. Additionally, we discuss how surgery may one day represent an important management option for type 2 diabetes mellitus.