Interleukin-4 is a multifunctional cytokine that plays a critical role in the regulation of immune responses. Its effects depend upon binding to and signaling through a receptor complex consisting of the IL-4Rα chain and the common gamma chain (γc), resulting in a series of phosphorylation events mediated by receptor-associated kinases. In turn, these cause the recruitment of mediators of cell growth, of resistance to apoptosis, and of gene activation and differentiation. Here we describe our current understanding of the organization of the IL-4 receptor, of the signaling pathways that are induced as a result of receptor occupancy, and of the various mechanisms through which receptor function is modulated. We particularly emphasize the modular nature of the receptor and the specialization of different receptor regions for distinct functions, most notably the independent regulation of cell growth and gene activation.