Leptin is the product of the ob gene located in humans on chromosome 7q31.3. It is a 16-kDa protein named after the Greek "leptos," meaning lean, to indicate the function that this adipocyte-secreted protein was thought to have. Since its discovery, in fact, most of the research focused on the role of leptin in body-weight regulation, aiming to elucidate the pathophysiology of human obesity. However, more and more data show that leptin is not only important in the regulation of food intake and energy balance, but it also functions as a neuroendocrine hormone. It is involved in glucose metabolism, as well as in normal sexual maturation and reproduction, and interacts with the hypothalamic-pituitary-adrenal (HPA) and the growth hormone (GH) axes.