GPCRs (Guanine Nucleotide Binding–Protein Coupled Receptors) comprise large and diverse gene families in fungi, plants, and the animal kingdom. Also termed serpentine receptors, GPCRs are polytopic membrane proteins that share a common structure with seven transmembrane segments, but sequence similarity is minimal among the most distant GPCRs. Their principal function is to transmit information about the extracellular environment to the interior of the cell, and they do this by interacting with the G-proteins. 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. These receptors affect the generation of small molecules that act as intracellular mediators or second messengers, and can regulate a highly interconnected network of biochemical routes. The intracellular signaling pathways activated by GPCR signaling include cAMP/ PKA pathway, Ca2+/PKC pathway, Ca2+/NFAT pathway, PLC pathway, PTK pathway, PKC/MEK pathway, p43/p44MAPK pathway, p38 MAPK pathway, PI3K pathway, NO-cGMP pathway, Rho pathway, NF-κB and JAK/STAT pathway.