Endothelial cells can be the prime target for an infection and infected endothelial cells may serve as an initiating system for a systemic response as these cells are able to secrete many mediators known to be of paramount importance. Endothelial cell functions in turn are regulated by these circulating mediators. Cellular interactions with leukocytes revealed protective and destructive functions. Single cell and animal studies indicate that endothelial permeability is increased and apart from clinical obvious edema formation in septic patients, the endothelial component remains unknown. Endothelial coagulation activation has been shown in vitro, however human data supporting an endothelial procoagulatory state are lacking. Defects in endothelium dependent vasoregulation in animal models are well known and again human studies are largely missing. An imbalanced production of reactive oxygen species including nitric oxide has been found to be involved in all endothelial functions and may provide a common link which at present can be supported only in animal studies.