The obesity epidemic is a major health problem that is associated with increased morbidity and mortality. Gastrointestinal hormones have been increasingly understood to be an important element in appetite regulation. Several gastrointestinal hormones can contribute to obesity by modulating the activity of the gut-brain axis. Bariatric surgery is currently the most effective therapy for significant and sustained weight loss in morbidly obese patients. Understanding how gut hormones are altered by bariatric procedures has contributed to our understanding of the mechanisms of appetite. In this review, we address several gastrointestinal hormones that are associated with obesity and consider how their levels are altered after bariatric surgery. The review also addresses specific effects of different gut hormones on appetite, hunger, and energy balance.