The growth of the opiate field over the past decade has been enor mous. Initial interest focused upon the strategic clinical impor tance of morphine and its analogs, but the discovery of the enkephalins and the other endogenous opioid peptides with their widespread actions within brain has expanded the field to investi gators in almost all areas of neuroscience as well as pharmacol ogy. Unfortunately, this field of research with its vast literature has become progressively more complex. The receptors are no longer limited to opiates, but include many subtypes selective for the opioid peptides. Indeed, they might be better termed opioid, rather than opiate, receptors. Many controversies have emerged and been settled; others remain. Early studies must now be inter preted on the basis of current information. Thousands of papers examining various aspects of opiates and the endogenous opioids present separate pieces of a large puzzle. The goal of this volume is to put the pieces together to give a coherent overview of opiate receptor pharmacology and to provide insights into both the mo lecular and classical pharmacology of opiates and the opioid pep tides. The issue of multiple classes of opiate and opioid peptide re ceptors and their importance in understanding mechanisms of ac tion provides the major focus of the book. The study of opiates and opioid peptides provides a unique research opportunity in the neuropharmacology of drug receptors.