The melanocortin system provides a conceptual blueprint for the central control of energetic state. Defined by four principal molecular components--two antagonistically acting ligands and two cognate receptors--this phylogenetically conserved system serves as a prototype for hierarchical energy balance regulation. Over the last decade the application of conditional genetic techniques has facilitated the neuroanatomical dissection of the melanocortinergic network and identified the specific neural substrates and circuits that underscore the regulation of feeding behavior, energy expenditure, glucose homeostasis and autonomic outflow. In this regard, the melanocortin-4 receptor is a critical coordinator of mammalian energy homeostasis and body weight. Drawing on recent advances in neuroscience and genetic technologies, we consider the structure and function of the melanocortin-4 receptor circuitry and its role in energy homeostasis.