Endothelial cells play a wide variety of critical roles in the control of vascular function. Indeed, since the early 1980s, the accumulating knowledge of the endothelial cell structure as well as of the functional properties of the endothelial cells shifted their role from a passive membrane or barrier to a complex tissue with complex functions adaptable to needs specific in time and location. Hence, it participates to all aspects of the vascular homeostasis but also to physiological or pathological processes like thrombosis, inflammation, or vascular wall remodeling. Some of the most important endothelial functions will be described in the following review and more specifically, their role in blood vessel formation, in coagulation and fibribolysis, in the regulation of vascular tone as well as their participation in inflammatory reactions and in tumor neoangiogenesis. J. Cell. Physiol. 196: 430–443, 2003. © 2003 Wiley‐Liss, Inc.