Neuropeptide Y (NPY) is a 36 amino acid peptide originally isolated from porcine brain which has now been shown to be one of the most abundant and widespread peptides in the mammalian nervous system. NPY-like peptides have been reported from non-mammalian vertebrates and from some invertebrates. Mammalian NPY shows striking sequence similarity to other biologically active peptides most notably PYY and pancreatic polypeptide. The human NPY precursor encodes a single copy of NPY and a 30 amino acid peptide located at the C-terminal referred to as C-PON. Both peptides are co-localised, and presumably secreted, but no role has yet been proposed for C-PON as an effector of biological function. In contrast, NPY is a multi-functional peptide, which is widely accepted to play a major role in the central control of appetite, feeding behaviour and energy balance, and to exert coronary vasomotor and inotropic effects in the mammalian heart. At least three receptor types capable of binding NPY and NPY fragments have been identified from several tissues. NPY has been co-localised with many other transmitters, including catecholamines, nitric oxide and other neuropeptides, with various combinations reported from single cell types.