Publisher Summary This chapter discusses the guanine nucleotide-binding regulatory proteins (G proteins) and the early events of platelet activation. G proteins are heterotrimeric proteins that mediate the interaction between cell surface receptors and cellular effectors, including second messenger generating enzymes and ion channels. Although G proteins are present in all mammalian cells, different types of cells differ in their complement of G proteins. To date, nine different G proteins have been identified in platelets, including Gq, G11, G12, G13, Gz, three members of the Gi family, and at least one variant of GS. Their known targets include phospholipase C, phospholipase A2 and adenylyl cyclase. Efforts to clone the receptors that can couple to these G proteins have been successful for epinephrine, thrombin, thromboxane A2, vasopressin and platelet activating factor. In each case, the receptor has a characteristic structure composed of a single polypeptide with seven transmembrane domains and an extracellular N-terminus. Receptor activation typically occurs upon agonist binding, but in the case of the thrombin receptor, activation involves a novel mechanism in which thrombin cleaves its receptor, creating a new N-terminus that serves as a tethered ligand. Current efforts to understand the role of G proteins in platelets focus upon the identification of which G proteins interact with specific receptors and defining the role that G proteins may play in megakaryocyte development, platelet formation and the secretion of platelet storage granules.