The G protein-coupled receptor (GPCR) superfamily is the largest family of integral membrane proteins. GPCRs respond to a wide variety of sensory and chemical stimuli and contribute directly to the regulation of all major organ systems. As such, GPCRs represent primary drug targets for therapeutic intervention. Although GPCRs respond to a diverse range of ligands and signal through multiple heterotrimeric G proteins, the inactivation of GPCR signaling is mediated by a limited set of proteins. In particular, the desensitization of the majority of GPCRs is mediated by the binding of two arrestin isoforms, beta-arrestin1 and beta-arrestin2, that exhibit overlapping substrate specificity. In response to GPCR activation and phosphorylation by GPCR kinases, beta-arrestins redistribute from the cytosol to the plasma membrane to bind GPCRs and subsequently target the receptors for internalization via clathrin-coated vesicles. This property of beta-arrestins has allowed the development of a green fluorescent protein (GFP)-based assay for detecting GPCR activation by confocal microscopy. This beta-arrestin-GFP translocation methodology is described in detail in this chapter.