The strategic location of the endothelium allows it to detect changes in hemodynamic forces and blood-borne signals, and to respond by releasing a number of autocrine and paracrine substances. The balanced release of these bioactive factors facilitates vascular homeostasis. If disrupted, endothelial cell dysfunction ensues. This predisposes the vessel wall to vasoconstriction, leukocyte adherence, platelet activation, thrombosis, vascular inflammation and atherosclerosis. Given the central role of the endothelium in the development and progression of atherosclerosis, endothelial function testing may serve as a useful biomarker of atherosclerotic disease. The present review highlights the current modalities used in assessing endothelial function, explores how endothelial function may serve as a biomarker for atherosclerosis, comments on the prognostic relevance of endothelial function and describes its use in the clinical setting.