The arterial endothelium is a complex organ that modulates vascular tone by release of various substances to control perfusion. Endothelial function reflects vascular ageing and health. Already at the earliest stages of atherosclerosis the delicate balance between arterial constriction and relaxation is disturbed. Therefore, non-invasive assessment of endothelial function is a means to identify patients at increased cardiovascular risk, even at levels of disease that cannot be identified with classical imaging techniques that depict arterial wall and/or lumen or with functional assessment of ischaemia. Currently, there is an increasing interest in the early recognition of endothelial dysfunction to streamline and optimise preventive therapeutic measures. In this article, several methods for the assessment of endothelial function are briefly reviewed. In particular, we discuss the fast bed-side assessment of endothelial function by the reactive hyperaemia peripheral arterial tonometry (RH-PAT) method.