Neuropeptide Y (NPY) and the related receptors represent a widely diffused system that is involved in the regulation of multiple biological functions. NPY, a 36-aminoacid peptide expressed in several areas of the nervous system, is a pleiotropic factor participating to the control of some physiological processes, such as cognitive functions, eating behavior, circadian rhythms, neuroendocrine mechanisms, reproductive and cardiovascular functions. NPY acts through a series of G-protein-associated membrane receptors (NPY-Rs), characterized by different tissue distribution and affinity for the ligand. The expression and secretion of NPY and the expression of NPY-R isoforms are controlled by a very wide range of agents, acting in an endocrine and / or paracrine fashion. NPY and NPY-Rs appear to be strongly involved in the control of eating behavior, their expression is modulated by changes of food intake and energy balance and is disrupted in several animal models of obesity and diabetes. Moreover, the hypothalamic NPY system appears to integrate signals of energy balance in the modulation of the reproductive axis. Agents that stimulate their expression include activators of intracellular signalling pathways (protein kinase A and C), classical neurotransmitters, steroid and peptide hormones and growth factors, while other agents (leptin, insulin and retinoic acid) have been shown to be inhibitory. Interestingly, some agents, like retinoic acid, have been shown to modulate the expression of both NPY and NPY-Rs in the same direction, thus providing a fine mechanism for the tuning of the system. The regulation of NPY / NPY-R expression and function appears to be part of a complex system controlling multiple physiological functions, and its disruption might be relevant in the pathophysiology of disease states such as obesity.