Obesity increases the risks of metabolic disorders including type 2 diabetes mellitus (T2DM). Bariatric surgery is the most successful therapeutic option that causes sustained weight loss and improvements in obesity comorbidities. Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG) are two of the most frequently performed bariatric surgeries. Despite their different anatomical rearrangement, they have remarkably similar success in both weight loss and T2DM remission. Interestingly, they also both cause a wide range of endocrine changes. Many of these endocrine changes are reflected specifically within the intestine and are implicated as mechanisms for the metabolic success of surgery. However, while most of the work shows that these hormonal changes are associated with the metabolic changes after surgery, causation has been difficult to ascertain. Here, we review the endocrine changes after RYGB and VSG and explore their mechanistic role in the success of bariatric surgery. Further, we explore important changes in gastrointestinal function and the role of these changes in the increase in postprandial endocrine responses after bariatric surgery.© 2017 American Physiological Society. Compr Physiol 7: 783-798, 2017.