Type 2 diabetes mellitus affects more than 170 million people worldwide. Because this disease is strongly linked to obesity, the term "diabesity" has been coined to describe the confluence of the 2 disease processes. Bariatric surgery has been performed for many years to achieve sustained weight loss in the morbidly obese population. As a secondary effect, a remarkable improvement in glycemic control is commonly achieved postoperatively. This has led to substantial interest in the use of bariatric procedures to treat type 2 diabetes mellitus. Surgical procedures in common use include the adjustable gastric band, the Roux-en-Y gastric bypass, the biliopancreatic diversion with duodenal switch, and the sleeve gastrectomy. Additionally, several investigational procedures including the ileal interposition and duodenal-jejunal bypass have been proposed as primary interventions for type 2 diabetes mellitus. These operations achieve their metabolic effects through a combination of volume restriction, intestinal bypass, and hormonal changes. As more data become available on the positive effect of bariatric procedures on type 2 diabetes mellitus, the use of such operations may grow. Bariatric surgery may ultimately become a major tool in the long-term treatment of type 2 diabetes mellitus. This manuscript presents an extensive review of the literature supporting these concepts.