Over the last 30 years, our understanding of a receptors has undergone a colossal evolution. They began as theoretical entities, then progressed to enigmatic receptors, and finally to identified proteins with important biological functions. Since the first book on a receptors was published in 1994, there have been many significant advances in the field. We now know that o receptors subserve many critical functions in the body and recent studies indicate that they are promising drug development targets for a host of neurological, psychiatric, cardiovascular, ophthalmological, immunological, and gastrointestinal disorders. This book provides a timely update on the medicinal chemistry, cell biology, and clinical implications of a receptors. It puts the information in a historical perspective to help new comers to the field successfully navigate the confusing early history surrounding these proteins, and it provides a launching point from which future studies and research directions can easily be developed. The full impact of a receptors on biological function has yet to be determined. The existing gaps in our knowledge base offer untold opportunities for future research. It is our hope that the information contained in this book will stimulate new, exciting research on a receptors and ultimately lead to innovative insights into basic biological mechanisms and novel therapeutic advances.