Bariatric surgery leads to substantial and durable weight reduction. Nearly 30% of patients who undergo bariatric surgery have type 2 diabetes, and for many of them, diabetes resolves after surgery (84% to 98% for bypass procedures and 48% to 68% for restrictive procedures). Glycemic control improves in part because of caloric restriction but also because gut peptide secretion changes. Gut peptides, which mediate the enteroinsular axis, include the incretins glucagon-like peptide-1 and glucose-dependent insulinotropic peptide, as well as ghrelin and peptide YY. Bariatric surgery (particularly bypass procedures) alters secretion of these gut hormones, which results in enhanced insulin secretion and sensitivity. This review discusses the various bariatric procedures and how they alter the enteroinsular axis. Familiarity with these effects can help physicians decide among the different surgical procedures and avoid postoperative hypoglycemia.