Until recently, it has largely been assumed that G protein-coupled receptors (GPCRs) function as monomeric entities. However, over the past few years, we and others have documented that GPCRs can form dimers and oligomers, leading to a re-evaluation of the mechanisms thought to mediate GPCR function. Despite the growing number of investigations into dimerization, little is known about the structural basis of receptor-receptor interactions and the functional consequences of dimer formation. Here, we present a brief review of some insights we have gained into the dimerization of dopamine and serotonin receptors. We have demonstrated that agonist-regulated trafficking is identical for receptor monomers and dimers, however, agonist treatment appears to stabilise the receptor oligomers. An investigation of the structural assembly between receptors involved in dimerization showed that there are several sites of interaction including hydrophobic transmembrane domain interactions and intermolecular disulphide bonds. We have also examined receptor hetero-oligomerization and demonstrated the potential for novel functions as a result of these associations. Finally, as a result of these observations, we have been able to present evidence that GPCRs function as oligomers in the cell.