It now appears that leptin is the peripheral signal, hypothesized in the lipostasis theory, that informs the central nervous system how much adipose tissue there is in the body. The ability of the leptin signal to regulate body composition and the amount of body fat has been demonstrated in animals. Furthermore, defects in the ob gene and the leptin receptor lead to the development of obesity in rodents. No such defects have been found in humans although it appears that obese humans are resistant to the action of their endogenous leptin. Further characterization of the hormone and ultimately, the administration of leptin to humans, will be necessary to determine the role of the leptin signal system in the development of obesity in humans.