Several studies have concluded that Roux-en-Y gastric bypass surgery (RYGBS) is the best current treatment option for obesity-related Type 2 Diabetes Mellitus (T2DM). The mechanisms underlying RYGBS-induced improvement in glycemic control remain unclear. Many investigators have advocated that this effect does not depend upon body weight loss, based on clinical observations that improvement in glucose homeostasis occurs early in the postoperative period, often prior to hospital discharge. Understanding the mechanisms underlying the metabolic effects of RYGBS will help to engineer ways to improve RYGB or to produce these effects without surgery.