For more than twenty years now, GPCR dimers and larger oligomers have been the subject of intense debate in the field. Evidence for a role of such complexes in receptor trafficking to and from the plasma membrane have been provided, but one of the main issue is of course to determine whether or not such a phenomenon can be responsible for reciprocal control (allosteric control) of the subunits. Such a possibility would indeed add to the possible ways a cell can integrate various signals targeting GPCRs. Among the large GPCR family, those of the class C that include mGlu and GABAB receptors, represent excellent models to examine such a possibility as they are mandatory dimers. In the present review, we will report on the observed allosteric interaction between the subunits of class C GPCRs, both mGluRs and GABABRs, and on the structural bases of these interactions. We will then discuss these findings for other GPCR types such as the rhodopsin-like class A receptors. We will show that many of the observations made with class C receptors have also been reported with class A receptors, providing interesting evidence that such allosteric interactions can be of physiological relevance.