Leptin is a hormone that is produced by adipose tissue and plays critical roles in the physiologic regulation of body weight. Leptin acts through the leptin receptor (LEPR) to regulate adipose mass by inhibiting hunger and balancing energy usage. Leptin mutations cause severe hereditary obesity and hypogonadism in rodents and humans. Leptin also has thermogenic actions, regulates enzymes of fatty acid oxidation, and is involved in hematopoiesis, angiogenesis, wound healing, inflammation, and immune responses.