This chapter focuses on one family of novel genes identified via the use of genomic techniques, the family of orphan G protein-coupled receptors (GPCRs), and describes the methods one can use to try and characterize these and identify their native ligands. The choice of functional assays used for screening orphan receptors is crucial to success. The assays should be as generic as possible in order to pick up a wide range of coupling mechanisms. Although the sequence homology with known receptors is low, orphan receptors would first be screened against a bank of known or putative GPCR ligands when “fishing” for the cognate ligand. Fluorescence imaging plate reader technology has provided a major improvement in throughput and sensitivity when screening orphan receptors but, as with many new technologies, additional technical problems arise, particularly when using the system to screen extracts of biological tissues.