Obesity is an important risk factor for type 2 diabetes (T2D). Weight loss improves the major factors involved in the pathogenesis of T2D, namely insulin action and β-cell function, and is considered a primary therapy for obese patients who have T2D. Unfortunately, most patients with T2D fail to achieve successful weight loss and adequate glycemic control from medical therapy. In contrast, bariatric surgery causes marked weight loss and complete remission of T2D in most patients. Moreover, bariatric surgical procedures that divert nutrients away from the upper gastrointestinal tract are more successful in producing weight loss and remission of T2D than those that simply restrict stomach capacity. Although upper gastrointestinal tract bypass procedures alter the metabolic response to meal ingestion, by increasing early post-prandial plasma concentrations of glucagon-like peptide 1 and insulin and increasing serum bile acid concentrations, it is not clear whether these effects make an important contribution to long-term control of glycemia and T2D once substantial surgery-induced weight loss has occurred. Nonetheless, the effects of surgery on body weight and metabolic function indicate that bariatric surgery should be part of the standard therapy for T2D. More research is needed to advance our understanding of the physiological effects of different bariatric surgical procedures and possible weight loss-independent factors that improve metabolic function and contribute to the resolution of T2D.