Neuropeptides rank among the phylogenetically oldest interneuronal signal substances. In the concept of neuro-secretion they were identified as neurohormones by which - via the blood - the brain regulates peripheral functions. It is now evident that the neuropeptides act as neurotransmitters/-modulators, as (neuro-)hormones, and paracrine or autocrine signal substances in diverse parts of the body. This book reviews, in several comprehensive articles written by distinguished specialists, the state of the art in the field of neuropeptides and peptidergic neurons. Special topics concern molecular aspects of processing, release and degradation of neuropeptides, receptors and signal transduction, comparative and behavioural aspects, and immunoregulatory effects of neuropeptides and their involvement on pathology of the central nervous system.