Leptin was first characterized as a hormone that plays a central role in the control of body adiposity. A number of studies later revealed several other functions for leptin, including the capacity to modulate immune system activity. Currently, leptin occupies an important position as a unifying mechanism integrating nutritional status and immune function. Here, we will review some of the actions of leptin in the immune system, with special attention to the functions it exerts in the thymus.