The endothelium is a functional barrier between vessel wall and blood stream. Assuming the total human vascular and capillary system occupies a surface area of more than 1,000 m2 which is covered by 1,013 endothelial cells, the complex role of the endothelium for hemostasis and immunological and metabolic processes becomes obvious. Dysfunction of the endothelium is a critical factor in the pathogenesis of vascular diseases and thrombus formation. This paper provides a brief review of physiological endothelial functions and summarizes measurable changes in products released from endothelial cells under pathological conditions which were associated with endothelial dysfunction.