Receptor oligomerization was initially proposed as a mechanism by which epidermal growth factor activates the protein tyrosine kinase activity of its receptor. It is now well established that ligand-induced receptor oligomerization plays an important role in transmembrane signaling by a large number of receptors for hormones, cytokines and growth factors. Heterodimerization of the extracellular domains of two members of the same receptor family, or interaction with an accessory molecule, can increase the diversity of ligands recognized by individual receptors. Heterodimerization of cytoplasmic domains permits the recruitment of different complements of SH2-domain-containing signaling molecules, increasing the repertoire of signaling pathways that can be activated by a given receptor.