Examines current techniques for studying the receptors that activate one of the largest superfamilies of proteins when bound to specific ligands, especially methods related to studies that become possible by the molecular cloning of the receptors. Researchers working on drug discovery may particularly be interested in the section introducing the basic theory of receptor-ligand interactions and different kinds of ligand screening systems. The other sections cover the interactions of the receptors with G proteins and the receptor desensitization; and the three-dimensional structure of the receptors, which throws light on the interactions between them and the proteins and may lead to the theoretical prediction of effective ligands.