In the last two decades, our knowledge on regulatory peptides and their cognate receptors, most of which are members of the seven transmembrane receptor families, has increased enormously. Regulatory peptides are small proteins which, besides their hormonal functions in regulating cellular metabolism in various tissues, may also act as neurotransmitters, and thus they often carry the prefix "neuro". Many of the cognate receptors involved in transducing the peptidergic signal across the cell membrane via a familiy of G proteins exist in multiple forms, the number of which frequently exceeds that of the corresponding peptide ligands. In this book, various peptide-receptor systems are discussed, e.g CRF, somatostatin, TRH, opioid peptides, vasopressin, and oxyin. It also discusses new strategies such as "reverse physiology" to uncover new peptides and orphan receptors.