Ligand-binding assays are useful to identify new compounds that target GPCRs. Further analysis of the biological responses after compound binding will help complete the whole picture concerning the overall characteristics of the compound. Upon ligand binding, GPCRs change their conformation and activate coupled G proteins, which subsequently promote second messenger production via downstream effectors. By GPCR functional assays, we can detect the activity of target GPCR, and may provide additional information on functional selectivity of candidate compounds.