The term receptor in the context used currently in drug and hormonal studies is defined operationally as those molecules that specifically recognize and bind the drug or hormone and, as a consequence of this recognition, can lead to other changes, which ultimately result in biological response. Most drug and hormone receptors that are membrane bound are in the category of “integral” membrane proteins; that is, they are not readily solubilized from the membrane and require detergents or other hydrophobie bond-breaking agents to release them. Specific labeling that may be achieved by affinity labels is often needed in order to follow changes in the quantity or distribution of certain receptors. The application of the method to the identification and quantitation of receptors has been reviewed in this chapter. It details the purification, pharmacological studies, and biochemical structure-function studies. The chapter concludes that although affinity labeling has important contributions to make in solving the complex problems of receptor structure-function relationship, the technique must be used with caution since the perturbation caused by the presence of a covalently bound ligand is capable of altering the system under study in ways that are not as yet fully understood.