The glycoprotein (GP) Ib-IX-V complex is an abundant plasma membrane receptor that is expressed almost exclusively on platelets and megakaryocytes. It is highly specialized to carry out its most import important function: the initial adhesion of platelets to sites of vessel wall injury in regions of the vasculature where the wall shear stresses are very high. It does this by binding the plasma and vessel wall protein von Willebrand factor (VWF). Failure of these hemostatic mechanisms because of deficiency or dysfunction of either ligand or receptor greatly increases the risk for pathological bleeding. The complex also has other important functions, including facilitating the response of platelets to the important agonist thrombin, and involvement in inflammatory processes by interacting with neutrophil and monocyte integrin αMβ2. In this chapter, the structure, synthesis, and functions of the GPIb-IX-V complex are discussed.