Leptin, a protein hormone produced by the adipocytes, has long been recognized to regulate metabolism, neuroendorine and other physiological functions. Early findings of increased leptin production during infection and inflammation and dysregulated immune response in leptin signaling-deficient mice provide strong evidence for the involvement of leptin in the immune responses. Recent data have established the regulatory function for leptin in immunity similar to the function of a pro-inflammatory cytokine, while gene-targeting studies also demonstrated an essential role of leptin in regulating hematopoiesis and lymphopoiesis. Moreover, there has been increasing evidence that leptin is involved in the pathogenesis of various autoimmune diseases. This review discusses recent advances in understanding the role of leptin in immunity and leptin-signaling pathways involved in modulating immune homeostasis and autoimmune pathogenesis.