Food intake is regulated via neural circuits located in the hypothalamus. During the past decade our knowledge on the specific mediators and neuronal networks that regulate food intake and body weight has increased dramatically. An important contribution to the understanding of hypothalamic control of food intake has been the characterization of the ob gene product (leptin) via positional cloning. Absence of circulating, functionally active, leptin hormone results in massive obesity as seen in ob/ob mice. Leptin inhibits food intake and increases energy expenditure via an interaction with specific leptin receptors located in the hypothalamus. Leptin receptors, of which there are several splice variants (Ob-Ra through Ob-Re), belong to the superfamily of cytokine receptors, which use the JAK-STAT pathway of signal transduction. Obese db/db mice, which have a mutation in the db locus, are unable to perform JAK-STAT signal transduction due to absence of functionally active (long form; Ob-Rb) leptin receptors. Ob-Rb is primarily expressed in the hypothalamus, with particularly high levels in the arcuate, paraventricular, and dorsomedial nuclei and in the lateral hypothalamic area. The abundance of leptin receptors in the ventromedial and lateral hypothalamus supports early observations that these two regions are intimately associated with the regulation of food intake. Leptin receptors have been identified in neuropeptide Y (NPY)/lagouti-related peptide (AgRP)- and proopiomelanocortin (POMC)/cocaine- and amphetamine-regulated transcript (CART)-containing neurons of the ventromedial and ventrolateral arcuate nucleus, respectively, and in melanin-concentrating hormone (MCH)- and hypocretin/orexin-containing neurons of the lateral hypothalamus, suggesting that the above-mentioned messengers are mediators of leptin's action in the hypothalamus. Indeed, functional studies show that NPY, AgRP, POMC-derived peptides, CART, MCH, and hypocretins/orexins all are important regulators of food intake. Leptin is essential for normal body weight balance, but the exact mechanisms by which leptin activates hypothalamic neuronal circuitries is known to a limited extent. In order to find pharmaceutical approaches to treat obesity, further studies will be needed to reveal the exact mechanisms by which leptin lowers body weight and which role leptin and leptin receptors have in the pathogenesis of human obesity.