It is well established that the vascular endothelium consists of a single layer of endothelial cells (ECs) that lines the entire vascular system, which plays an important role in vascular homeostasis, immune and inflammatory reactions. The endothelial cells exert significant autocrine, paracrine and endocrine actions and influence smooth muscle cells, platelets and peripheral leucocytes. It represents an important barrier between the blood and the underlying interstitium and cells and has a major role in the regulation of vascular tone, producing several vasoactive mediators. This book examines up-to-date research regarding the role of endothelial functions in terms of atherosclerosis and its clinical manifestations.