G Protein Coupled Receptors (GPCRs) regulate a wide variety of normal biological processes and play a role in the pathophysiology of many diseases upon dysregulation of their downstream signaling activities. The intracellular signaling pathways activated by GPCR signaling include cAMP/ PKA pathway, PKC pathway, Ca2+/NFAT pathway, PLC pathway, PTK pathway, PKC/MEK pathway, MAPK pathway, p38 MAP pathway, PI3K pathway, Rho pathway, NF-κB pathway and JAK / STAT pathway. GPCRs regulate a wide variety of biological processes, such as vision, olfaction, the autonomic nervous system, and behavior. GPCRs recognize a variety of ligands and stimuli including peptide and non-peptide hormones and neurotransmitters, chemokines, prostanoids and proteinases, biogenic amines, nucleosides, lipids, growth factors, odorant molecules and light. Developing drug screening assays requires a survey of both target and related GPCR expression and function in the chosen cell-based model system as well as expression of related GPCRs to assess both direct and potential off-target side effects.