In addition to the orthosteric site, which recognizes endogenous ligands, most G protein-coupled receptors (GPCRs) possess topographically distinct allosteric sites that can be recognized by small molecules and accessory cellular proteins. Ligand binding to allosteric sites promotes a conformational change in the GPCR that can alter orthosteric ligand affinity and/or efficacy. Moreover, there has been an increase in recent years in the identification of allosteric agonists that can directly activate the receptor in the absence of orthosteric ligand. Allosteric sites are attractive therapeutic targets because they can be exploited to achieve modes of selectivity and signaling that are not attainable by orthosteric means. However, an important challenge in this field remains the quantification of the myriad of possible allosteric effects on binding and signaling events. This unit provides an overview on GPCR allosterism and the different pharmacological approaches to understanding allosteric behaviors. © 2010 by John Wiley & Sons, Inc.