During the last two decades, it has been become clear that vascular endothelium does not only represent a passive non-thrombogenic surface. but actively interferes in the coagulation process. It has been shown that the endothelium provides different cell membrane-associated components contributing to the maintenance of the anticoagulant state. Whereas some of these components have extensively been studied revealing major physiological significance, there is increasing evidence that additional proteins employed by the endothelium may play a substantial role with regard to hemostasis. Thus, the objective of this article is to overview all anticoagulant mechanisms that have been found to involve the endothelium. Considering the immense quantity of data available in this field of investigation, the molecular aspects are only briefly summarized, and other antithrombotic interactions, such as those with platelets and the fibrinolytic process, will be discussed elsewhere. The first part of this article describes the functional mechanisms, whereas the second part elucidates the physiological significance.