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Publisher Summary A large family of seven transmembrane-domain receptors for hormones, neurotransmitters, growth factors, chemoattractants, light, odorants, and other extracellular stimuli promote intracellular signaling responses by activation of heterotrimeric G proteins. This discrepancy between the rates of GTP hydrolysis measured in vitro and in vivo presaged the existence of proteins that accelerate the deactivation of GTP-bound Gα subunits. In the standard model of GPCR signaling, the duration of heterotrimeric G-protein signaling through effectors is thought to be controlled by the lifetime of the Gα subunit in its GTP-bound form. Regulator of G-protein Signaling (RGS) proteins promote the deactivation step in the heterotrimeric G-protein regulatory cycle, catalyzing GTPase rates in vitro that are consistent with the rates of deactivation of G-protein signaling in vivo. The physiological functions of these proteins continue to be investigated and, in the case of many RGS proteins, include additional functionalities beyond their hallmark capacity to act as GAPs for Gα subunits.