This paper concerns the adhesion of platelets to subendothelial structures, as measured in perfusion systems such as that developed by Baumgartner and his collaborators (1,2) and subsequently modified by Sakariassen et al (3,4). It has been shown that platelet adhesion to subendothelium is defective, particularly at high shear rates, in two hereditary bleeding disorders: Bernard-Soulier syndrome, in which the platelets lack the membrane glycoprotein Ib, and von Willebrand’s disease, in which the high-molecular-weight multimeric component of the factor VIII complex is either deficient from the plasma or structurally and functionally defective (5,6). I shall attempt here to review the growing body of evidence relating to the interactions between vessel wall components, the factor VIII/von Willebrand factor (vWF) and the platelet membrane, which are evidently necessary for adhesion to occur. I shall not have time for a detailed consideration of the relative importance in this series of reactions of the various components of the subendothelium, which remains a subject of much controversy, but shall confine myself to a discussion of the active sites involved on the platelets themselves and the vWF.