Heterotrimeric GTP-binding proteins (G-proteins) are a critical component of signal transduction pathways that carry information received at the cell surface to the appropriate cellular effector system, ultimately achieving a specific cellular response. Heterotrimeric G-proteins consist of an alpha-subunit, which contains the guanine nucleotide binding site and intrinsic GTPase activity, and an inseparable beta gamma-subunit complex. G-proteins act to define the specificity by which a receptor regulates a particular intracellular signaling system, as well as to regulate the duration of the signal. A great deal of structural and functional insight into how G-protein-mediated signal transduction occurs has recently been achieved. This review will discuss the structural features of G-proteins, as well as detail the mechanism by which G-proteins interact with receptors and effectors.