Over the last decade it has been shown that orphan G protein-coupled receptors (GPCRs) can be used as targets to discover novel neuropeptides. A dozen neuropeptides have been identified through this approach. Each of these neuropeptides has opened new doors for our understanding of fundamental physiological or behavioral responses. For example the orexins, MCH and ghrelin carry fundamental roles in regulating food intake while neuropeptide S, neuromedin S, the prokineticins and the orexins are major players in modulating sleep and circadian rhythms. The chapters of this book review the latest research in the field, most of them are written by the original discoverers of the respective novel neuropeptide. Emphasis is set not only on their discovery but also on their functional significance. Since many of these neuropeptides are part of drug discovery programs, this book impacts academic as well as pharmaceutical research.