This comprehensive collection of the major cross-disciplinary methods and protocols used in current opioid research covers topics ranging from molecular and genetic techniques, to behavioral analyses of animal models and to clinical practices. The well-practiced authors describe their best molecular techniques for the cloning and expression of opioid receptors, and for the quantitative characterization of their signaling pathways, as well as for mapping the distribution and detecting the expression levels of opioid receptors, opioid peptides, and their messages in both brain tissues and individual cells. Also included are methods for the creation of in vitro and in vivo animal models to study opioid functions, as well as the clinical applications in the treatment of pain and opioid addiction.