Obtaining a fuller understanding of gut hormones as mediators of appetite regulation and energy homeostasis has never been so important with obesity rates increasing at pandemic proportions. The role of the gut hormone peptide YY 3-36 (PYY3-36) in particular has sparked interest since the discovery of its anorectic effect in obese rodents and humans. Fasting circulating PYY concentrations correlate negatively with BMI and waist circumference in humans, whilst postprandial PYY levels predict subsequent changes in weight over a period of at least 6 months. Furthermore, Pyy null mice demonstrate increased adiposity and hyperphagia, which is reversed by exogenous PYY3-36. Chronic administration of PYY3-36 to diet-induced obese rodents has shown a dose-dependent reduction in adiposity. Taken in concert, these findings suggest that the PYY system may hold significant potential in the treatment and prevention of obesity.