Endothelial cells play a pivotal role in hemostasis and thrombosis. They produce a myriad of factors either associated with the membrane or released into the blood stream and the subendothelial matrix which are involved in various steps of hemostasis. The endothelial cell function is modulated by a diversified group of biologically active molecules, notably thrombin, vasoactive amines and cytokines. Mechanism and selectivity of the effects of these molecules differ and the difference may have important physiological implications. Most of the information is gathered through experiments performed in cultured endothelial cells. Availability of the cultured cells has greatly facilitated the understanding of endothelial cell biology. In vivo models, however, are still needed to understand how the endothelial cell function is modulated. Furthermore, as the cultured endothelial cells exhibit nor only species differences but also vascular origin difference in behavior and function, these factors should be carefully considered when designing experiments involving the use of cultured endothelial cells.