Protein allosterism is the change in protein reactivity at one site arising from a molecule binding on the protein at another site. Although allosterism traditionally has been discussed in terms of affinity changes of receptors, the increasing use of functional pharmacological assays makes it mandatory to consider effects on both the affinity and the efficacy. Antagonism of agonist response can occur allosterically by reduction of affinity and/or efficacy but the antagonist will have different properties depending on which of these is primarily affected. This paper discusses the collective behaviors of seven transmembrane (7TM) receptors as allosteric systems that have a modulator (ligand or protein) that interacts and transmits information through a conduit (receptor) to a guest (either other ligand, interacting protein or cytosolic protein). Such receptor allostery can be discussed as vectorial transfers of information from ligand-binding domains ('classical' modulator allosterism) to the cytosol (functional selectivity) and along the plane of the membrane (receptor dimerization).