Type 2 diabetes mellitus (T2DM) is a costly chronic disease that is associated with increased morbidity and mortality. Despite the development of numerous pharmacological agents, diet and exercise programs, and behavior-modification protocols, bariatric surgery remains the most effective tool for the durable reversal of T2DM. This review discusses the known effects of bariatric surgery on T2DM, with a particular focus on Roux-en-Y gastric bypass.