The regulation of body fat stores is a problem of energy and nutrient balance that can be most readily viewed as a feedback system. Several elements are involved in any feedback system, including afferent signals, a controller that senses the afferent signals and transduces their information and then activates efferent controls that regulate the controlled system. The recent discovery of leptin has provided a major missing link in the feedback control system. This afferent signal is produced exclusively in fat cells of nonpregnant mammals but can be produced in the placenta as well. This circulating peptide has a very strong relationship to the level of body fat and its absence experimentally and clinically produces massive obesity. In the controller, or brain, several anatomic regions play a central role in regulating fat stores. Damage to the ventromedial nucleus (VMH) or the paraventricular nucleus (PVN) in the hypothalamus produces massive obesity in mammals and birds. Injury to the central nucleus of the amygala will also produce obesity. In contrast, damage to the lateral hypothalamus reduces body fat. The syndrome of leptin deficiency or defects in the leptin receptors produce a massive obesity that is metabolically similar to the VMH or PVN lesion syndromes of obesity, suggesting that leptin may have its metabolic effects through these medial hypothalamic centers. Support for this idea has come from studies showing that damage to the PVN or VMH will block the effects of leptin. A number of neuropeptides and monoamines are involved with modulating of food intake and fat stores. Both serotonin, acting through 5-HT2C receptors, and norepinephrine, acting through beta 2 and/or beta 3 receptors, reduce food intake. A variety of peptides also influence food intake and body fat. Neuropeptide Y, dynorphin, galanin, and melanocyte-stimulating hormone all increase food intake. In contrast, a large number of peptides--including cholecystokinin, corticotrophin-releasing hormone/urocortin, enterostatin, insulin, leptin, alpha-MSH, and TRH--reduce food intake. Chronic administration of neuropeptide Y, acting through Y-5 receptors, can produce chronically increased food intake and obesity. This syndrome is similar to the VMH syndrome and suggests that NPY must be acting as an inhibitor of a feeding system. The melanocortin receptor system may be particularly important because a mouse that does not express MC4 receptors is massively overweight. These central systems modulate food intake and fat stores by the controlled system. Glucocorticoids from the adrenal gland are important in obesity, since adrenalectomy will reverse or prevent the development of all forms of obesity. The sympathetic nervous system is also important because low sympathetic activity is associated with experimental and clinical obesity. The reciprocal relationship between food intake and sympathetic activity has been a robust relationship, suggesting that beta receptors in the periphery or brain may be involved in feeding control. In one model of dietary obesity resulting when animals eat a high-fat diet, the syndrome is blocked by inhibitory adrenal steroid activity. These animals show a lower level of sympathetic activity and a low level of brain serotonin. Finally, they show an enhanced sensitivity to essential fatty acids when these are applied to the tongue or given into the gut. In this chapter, the control of energy stores as fat is viewed as a feedback system. Leptin is perceived as a key afferent signal and glucocorticoids and the sympathetic nervous system through beta receptors as essential elements of this control system.