Seven transmembrane receptors (7TMRs) are allosteric proteins that transmit information coded by ligands in the extracellular space, through a change in protein conformation, to signalling proteins in the cytosol. It is useful to refer to the agonist as the modulator, the 7TMR as the conduit and the signalling proteins as allosteric guests in this system since it will be seen that, in terms of the well-known property of probe dependence for allosteric ligands, a given modulator will have different effects on different cytosolic guests. This leads to agonist-specific signalling referred to as â functional selectivityâ or â biased agonismâ . This chapter discusses the discovery of these effects, their prevalence in pharmacology, their mechanism and possible therapeutic relevance.