The authors reviewed the most recent literature on leptin, a protein produced by adipocytes which exerts its action on hypothalamus, modifying eating behavior and inhibiting the lust for food consumption. This one appeared to be the main, if not the only, physiologic action of leptin. Later leptin has been acknowledged a major role in the homeostasis. The regulation of the synthesis, and the mechanisms by which the protein modulates both food intake and energetic balance have been evaluated, and the hypotheses on the regulatory function exerted by leptin on the homeostasis, by acting on neuroendocrine system, on sexual maturity and fertility, on the sympathetic nervous system, on hemopoiesis and hydroelectrolytic balance have been discussed, some of which being already supported by experimental evidences.