Body weight homeostasis is maintained via a series of complex interactions that occur between the brain (particularly the hypothalamus) and the periphery, notably via the hormone leptin, which is synthesized in and secreted from adipose tissue. Under normal conditions, a dynamic equilibrium exists between anabolic neuropeptides (orexigenic peptides), which favor food intake, decrease energy expenditure, and facilitate fat storage, and catabolic ones (anorexigenic peptides), which decrease food intake, increase energy expenditure, and facilitate the loss of fat stores. Secreted leptin, although it may have some direct peripheral effects, exerts its action principally within the brain. Following its transport through the blood-brain barrier, leptin reaches the hypothalamic area, where it binds to its long receptor isoform. After a specific signaling cascade, leptin inhibits many of the orexigenic neuropeptides while favoring many of the anorexigenic ones. Thus, leptin decreases food intake and body weight, and it increases fat oxidation and energy expenditure, ultimately favoring leanness. Lack of leptin secretion, the inability of leptin to reach the brain, or the inability of leptin to interact with hypothalamic leptin receptors, prevent leptin's effects and lead to obesity.