This review summarizes current insights in the importance of endothelium in the interaction of platelets with the subendothelium. A greater part of the results described are obtained with the use of cultured vessel wall cells. The following subjects are discussed: (1) the pro- and antithrombotic properties of the endothelial cells; (2) the synthesis of subendothelium by the endothelial cells and the influence of the composition of the subendothelium on its reactivity towards blood platelets; (3) the effect of (purified) matrix and plasma proteins on the adhesion process, and (4) various abnormalities of platelet adhesion are discussed that lead to enhanced bleeding tendency or to increased thrombus formation.