The prevalence of obesity is a significant public health issue in the United States, with 35.7%, 15.4%, and 6.3% of the U.S. population categorized with class 1 obesity (body mass index [BMI] ≥ 30 kg/m2), class 2 obesity (BMI ≥ 35 kg/m2), and class 3 obesity (BMI ≥ 45 kg/m2) respectively [1]. Obesity is associated with a number of comorbidities, including diabetes, hypertension, obstructive sleep apnea, cancer, cardiovascular disease, and decreased longevity [2]. Bariatric surgery has been the only intervention proved to provide durable long-term weight loss and effectively treat obesity-related comorbidities in patients with severe obesity [3, 4]. There is evidence that the weight loss and resolution of comorbidities after bariatric surgery go beyond caloric restriction, and malabsorption, and changes in gut physiology after some types of bariatric surgery may play an important role [5]. The gut is a complex endocrine organ whose functions include digestion and absorption processes, as well as control of satiety and insulin secretion via production of a hormonal response to the ingestion of food. Jejunoileal bypass and, more recently, Roux-en-Y gastric bypass (RYGB) have been long associated with enhanced release of gut satiety and incretin hormones that signal the central nervous system and the pancreas to control food intake and insulin secretion. The gut hormone changes after RYGB may favor satiety, decrease food intake, and sustain a new level of energy balance at a lower body weight that is approximately 30% lower than the maximum weight. The changes in incretin hormones after RYGB stimulate insulin secretion and help control blood glucose during meals. This chapter reviews the changes in gastrointestinal hormones found after the most common bariatric surgical procedures performed in the United States, specifically RYGB and vertical sleeve gastrectomy (VSG) [6]. We discuss the evidence, or lack thereof, for a possible role of these hormonal changes in the induction and maintenance of weight loss and diabetes remission.