Allosteric modulation of G protein-coupled receptors has been intensively studied at muscarinic acetylcholine receptors. Findings made with archetypal allosteric agents such as gallamine, alcuronium, and bis(ammonio)alkane-type agents revealed that binding of orthosteric ligands that attach to the acetylcholine site can be allosterically decreased or increased or left unaltered in a subtype-selective fashion. Analyses of structure/activity relationships (SARs) help to elucidate the molecular events underlying the allosteric action and they may pilot the development of new allosteric agents with improved properties and therapeutic perspectives. With a focus on SARs, this review illustrates the principles of muscarinic allosteric interactions, gives an overview of SARs in congeners of archetypal allosteric agents, and considers the topology of M(2) muscarinic allosteric interactions that are characterized by divergent binding modes.