Under normal conditions, the vascular endothelium exerts vasodilator, antithrombotic, and growth-inhibiting effects on the vessel wall. In the setting of atherosclerosis, endothelial dysfunction contributes to vasoconstriction, thrombosis, and growth of vascular smooth muscle. This article reviews the important contributors to endothelial function both in health and in the diseased state of atherosclerosis. Therapeutic options for improvement of endothelial function will also be summarized.