von Willebrand factor is a multimeric glycoprotein essential for the normal arrest of bleeding after tissue injury (hemostasis). The molecule is present in blood, both in plasma and inside platelets, as well as in endothelial cells and the subendothelial matrix of the vessel wall. Through multiple functional domains, von Willebrand factor mediates the attachment of platelets to exposed tissues, where discontinuity of the vascular endothelium occurs, and the subsequent platelet aggregation leading to the formation of platelet thrombi. The crucial role of von Willebrand factor in platelet function is particularly apparent when hemodynamic conditions create blood flow with high shear stress, as in capillaries under physiologic conditions or in stenosed and partially occluded arteries in disease states. The involvement of von Willebrand factor in the processes that lead to acute thrombosis has attracted considerable interest on the molecular and functional biology of the protein. Indeed, understanding the mechanisms and structural bases for von Willebrand factor function may result in new and effective approaches to anti-thrombotic intervention.