The classical physiological concept of the endothelium was that of a barrier and a surface of exchange. Little importance was attached to its role in the regulation of vascular tone. Successively, prostacycline, the endothelium-derived relaxing factor, the hyperpolarizing factor, the endothelium-derived contracting factor and especially endothelin, were discovered and their physiological effects are currently under evaluation. These different factors and their possible roles in vascular pathology are reviewed. The endothelial function or functions are probably abnormal in many pathological situations.