The adhesive protein von Willebrand factor mediates the initiation and progression of thrombus formation at sites of vascular injury. von Willebrand factor is synthesized in endothelial cells and megakaryocytes as a very large polymer composed of identical subunits. In the plasma, it appears as a series of multimers of regularly decreasing molecular mass, from several thousand to 500 kDa. The size of circulating von Willebrand factor multimers is controlled by proteolytic cleavage carried out by a specific protease. The biological functions of von Willebrand factor are exerted through specific domains that interact with extracellular matrix components and cell membrane receptors to promote the initial tethering and adhesion of platelets to subendothelial surfaces, as well as platelet aggregation. Moreover, von Willebrand factor binds the procoagulant co-enzyme, factor VIII, contributing to its stability and, indirectly, to its function in the generation of fibrin. This chapter presents a review of current knowledge on the structure, biosynthesis and functions of von Willebrand factor.