Leptin, a 16 kD protein and a product of the obesity (ob) gene, is synthesized and secreted predominantly by adipose tissue and is, therefore, considered an adipokine. With circulating levels directly proportional to the total amount of body fat, leptin plays an important role in whole body metabolism, regulating food intake and energy balance, as well as assisting in the control of homeostasis, reproductive function, the immune system and numerous other body functions. Binding to receptors of the hypothalamus, leptin signals food satiety and reduces appetite, although evidence suggests that, in general, obese people may have unusually high circulating levels of leptin. This may indicate a resistance to leptin similar to the insensitivity to insulin associated with type 2 diabetes. Consequently, the study of leptin is essential to understanding the relationship among appetite, energy balance and obesity.