Heterotrimeric G proteins, commonly known as G proteins are members of the large GTPase superfamily that are conserved from bacteria to mammals. G proteins play an essential role in cellular signaling by coupling many cell surface receptors to effectors on the plasma membrane. G proteins are composed of an alpha-subunit that binds and hydrolyzes GTP and a beta gamma-subunit complex. To date over 20 different a subunits, 5 beta subunits and 14 gamma subtypes have been defined. Classification of G protein heterotrimers are based on the primary sequences of their alpha-subunits, resulting in four main families. In its inactive state, each G protein is a heterotimer in which the GDP bound alpha chain is tightly associated with the beta gamma subunits. Receptor activation promotes the replacement of GDP by GTP and the consequent dissociation of a from beta gamma. Both alpha GTP and the beta gamma complex interact with and regulate specific effectors. Activation is then terminated by the intrinsic GTPase activity of the a chain, which returns the protein to its inactive state. G proteins are regulated by posttranslational modifications including phosphorylation, myristoylation, palmitoylation, and prenylation of G alpha- and G beta gamma- subunits. G protein alpha- and beta gamma- subunits regulate the activities of diverse effectors. G proteins mediate a wide variety of cellular responses, including sensory perception, neuronal activity and hormonal regulation. Recent reports indicate that GPCRs and G proteins are also involved in the regulation of cell growth, differentiation and cellular transformation.