The endothelium is a monolayer of cells that comprises the luminal surface of all blood and lymphatic vessels. Interposed between the flowing blood or lymph, the endothelium regulates the interaction of the vessel wall with circulating cells, platelets, exosomes, and humoral factors. Endothelial adhesion molecules and chemokines facilitate leukocyte traffic into tissues. A panoply of endothelial paracrine factors modulates the tone and growth of the underlying vascular smooth muscle. Paradigmatic of these paracrine factors is endothelium-derived nitric oxide (NO), a potent vasodilator that also suppresses proliferation of vascular smooth muscle cells and infiltration of leukocytes. 1 The generation of NO is also essential for endothelial cell proliferation and migration and critical for angiogenesis.