The vascular endothelium has long been considered a passive inner lining of blood vessels. But in recent decades several observations have proved its involvement in vital functions of the cardiovascular system, including regulation of perfusion, fluid and solute exchange, haemostasis and coagulation, inflammatory responses, vasculogenesis and angiogenesis4-7. This involvement has further been proven to play a profound role in the pathogenesis of cardiovascular disease1, and therefore a parameter to measure endothelial function is in growing demand. Several studies have suggested that the parameter FMD (flow-mediated dilation) could be clinically applied in such measurements121,122,124. However the clinical significance of FMD in the general population is still under investigation. In order to apply this parameter to the general population, one must find ways to standardize different aspects of measurement and generate reference ranges. In this presentation we consider the relevant anatomical and physiological aspects of vascular endothelium, and its role in different aspects of cardiovascular disease. The strengths and weaknesses of FMD are highlighted, before we conclude with a brief presentation of an example of how to clinically measure FMD.