A healthy endothelium maintains vascular tone and structure by regulating the balance between vasodilation and vasoconstriction, growth inhibition and growth promotion, antithrombosis and prothrombosis, anti-inflammation and proinflammation, and also antioxidation and pro-oxidation. The vascular endothelium is involved in the release of various vasodilators, including nitric oxide (NO), prostaglandins, and endothelium-derived hyperpolarizing factor as well as vasoconstrictors. NO plays an important role in the regulation of vascular tone, the inhibition of platelet aggregation, and the suppression of smooth muscle cell proliferation. Cardiovascular diseases are associated with endothelial dysfunction. In addition, endothelial dysfunction is an early feature of atherosclerosis and vascular diseases in humans. Therefore, it is clinically important to estimate the degree of endothelial dysfunction. Several methods have been used to assess endothelial function in humans. Measurement of flow-mediated vasodilation (FMD) using an ultrasound is noninvasive and is an accurate indicator of NO production. Measurement of FMD is widely performed as an index of conduit artery endothelial function. Several investigators have shown that FMD is an independent predictor for cardiovascular outcomes. It is thought that FMD is useful for surrogate endpoint of cardiovascular outcomes and index of atherosclerotic treatment. However, the evaluation technique by the FMD measurement system still requires skills in measurement of the vessel diameter by an ultrasound system, and is difficult to handle. In addition, there is a problem in that the technique requires a large-scale apparatus and lacks in simplicity. In this session, we would like to explain in detail concerning the methods of assessing and clinical importance of endothelial function, especially FMD.