Arrestins were originally defined as terminators of G protein-coupled receptor (GPCR) signaling, yet structural and mechanistic advances now reveal them as programmable, spatiotemporal integrators of cellular signaling. Recent cryo-electron microscopy studies have revealed a diverse spectrum of GPCR-arrestin engagement modes, including core-, tail-, loop-, side-engaged, and membrane-anchored conformations, across GPCR classes and arrestin isoforms. These structures reveal that arrestin recruitment operates as a conditional, allosterically regulated process rather than a binary on-off switch. The selection of the arrestin microstate is governed by layered regulatory inputs, including GPCR kinase-dependent phosphorylation barcodes, membrane and lipid cofactors, and isoform-specific mechanics, which together define the signaling geometry, duration, and subcellular localization. This structural logic provides a mechanistic foundation for biased signaling, noncanonical endosomal signaling, and GPCR-independent arrestin functions. Importantly, emerging therapeutic strategies, including intracellular allosteric modulators and molecular glues, demonstrate that arrestin signaling can be reprogrammed by directly sculpting transducer assemblies rather than ligand efficacy alone. Here, we synthesize recent structural, biochemical, and physiological insights to outline how arrestins decode regulatory inputs into signaling outcomes and how this knowledge enables the development of next-generation, structure-guided GPCR therapeutics.