G protein-coupled receptors (GPCRs) are key proteins that help transmit extracellular signals into cells. Arrestin molecules help regulate GPCR signaling by recognizing and binding to GPCR residues that have been phosphorylated specifically by the kinase GRK. Two models, the barcode model and the flute model, have been proposed to explain this process. In the barcode model, different protein kinases produce different phosphorylation “barcodes” on GPCRs and arrestins “read” the barcodes produced by GRK to produce certain signaling outcomes. In the flute model, different phosphorylation patterns form different sequences of “notes”. These notes can then be “played” in various ways by the different structural features of arrestins, enabling multiple “songs” (outcomes) to be produced from one set of notes. However, a new model, the QR code model, better captures the complexity of GPCR–arrestin signaling by integrating the influences of multiple factors, including ligands, phosphorylation sites, arrestin structures, and GRK, on signaling outputs. Although this model doesn’t apply to all types of GPCR signaling, it provides an updated and theoretical view of GPCR–arrestin signaling pathways and may help researchers design drugs that target these important pathways to modify biological processes.