This book covers the latest knowledge in structure, signaling, and biochemical pharmacology of KOR as well as preclinical research and clinical applications (including clinical phase studies and approved for human use) of KOR compounds. It is divided up into the three parts: Molecular aspects of KOR, Preclinical research on pharmacology of KOR agonists and antagonists in animals and KOR agonists and antagonists in clinical use and in past and present clinical trials.