Although classical models predict that G-protein-coupled receptors (GPCRs) function as monomers, several recent studies acknowledge that GPCRs exist as dimeric or oligomeric complexes. In addition to homodimers, heterodimers between members of the GPCR family (both closely and distantly related) have been reported. In some cases heterodimerization is required for efficient agonist binding and signaling, and in others heterodimerization appears to lead to the generation of novel binding sites. In this article, the techniques used to study GPCR heterodimers, and the 'novel pharmacology' and functional implications resulting from heterodimerization will be discussed.