Obesity has become a severe epidemic globally. With the rapid increase in obesity comes a pronounced rise in obesity-related disorders, especially type 2 diabetes. Weight loss achieved through lifestyle changes improves diabetes, but is very difficult to maintain over time. In contrast, Roux-en-Y gastric bypass (RYGB) surgery can induce long-term weight loss and remission of diabetes. Surprisingly, the resolution of type 2 diabetes occurs within days after surgery and before significant weight loss. The resolution of diabetes must therefore be explained by a response to the surgical alterations of the stomach and intestines: restriction in gastric volume and bypass of the stomach, duodenum and proximal jejunum.