Gastric bypass surgery (GBP) results in important and sustained weight loss and remarkable improvement of Type 2 diabetes. The favorable change in the incretin gut hormones is thought to be responsible, in part, for diabetes remission after GBP, independent of weight loss. However, the relative role of the change in incretins and of weight loss is difficult to differentiate. After GBP, the plasma concentrations of the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide increase postprandially by three- to fivefold. The postprandial incretin effect on insulin secretion, blunted in diabetes, improves rapidly after the surgery. In addition to the change in incretins, the pattern of insulin secretion in response to oral glucose changes after GBP, with recovery of the early phase and significant decrease in postprandial glucose levels. These changes were not seen after an equivalent weight loss by diet. The improved insulin release and glucose tolerance after GBP were shown by others to be blocked by the administration of a GLP-1 antagonist, demonstrating that the favorable metabolic changes after GBP are, in part, GLP-1 dependent. The improved incretin levels and effect persist years after GBP, but their long-term effect on glucose metabolism, and on hypoglycemia post GBP are yet unknown. Understanding the mechanisms by which incretin release is exaggerated postprandially after GBP may help develop new less invasive treatment options for obesity and diabetes. Changes in rate of eating, gastric emptying, intestinal transit time, nutrient absorption and sensing, as well as bile acid metabolism, may all be implicated.