β-Arrestins are ubiquitously expressed in all cell types, and function in the desensitization of G- protein coupled receptors (GPCRs), the control of GPCR intracellular trafficking, and the activation of GPCRs to multiple signaling pathways (1-4). Therefore, β-arrestin-mediated signaling constitutes an important part of GPCR signaling in addition to G protein-mediated signaling. As many GPCRs are found to recruit β-arrestin, the β-arrestin recruitment assay has found important use in drug discovery, especially in the discovery of ligands for orphan GPCRs and in situations where the second messenger signaling is unknown (5,6). Furthermore, the discovery of biased GPCR ligands and the findings that distinct G-proteins versus β-arrestin signaling preferences may offer therapeutic advantages over conventional ligands imply that a screening campaign should be designed to focus on the most disease relevant pathways (7-10). In this aspect, the β-arrestin recruitment assay has added an important piece to the repertoire of assay tools in drug discovery. There are four major in vitro assay technologies available on the market that are capable of measuring ligand-induced β-arrestin recruitment: PathHunter β-arrestin Assay (DiscoverX) (11), Tango GPCR Assay (Thermo Fisher Scientific) (12), LinkLight GPCR/ β-arrestin Signaling Pathway Assay (BioInvenu) (13), and Transfluor Assay (Molecular Devices) (14). The PathHunter β-arrestin Assay, Tango GPCR Assay System and LinkLight GPCR/ β-arrestin Signaling Pathway Assays are homogenous, high throughput assays while the Transfluor Assay is a fluorescence image-based assay. All four assays involve the expression of the β-arrestin as a fusion protein with another protein or fragment, while the PathHunter, Tango and LinkLight assays require fusion of the GPCR to another peptide or protein moiety as well. Table 1 compares the principles of these technologies as well as their advantages and limitations. This guideline uses PathHunter β-arrestin from DiscoverX to illustrate the concepts for performing GPCR β-arrestin recruitment assay. The principle behind this guideline can be applied to all the β-arrestin assay technologies.