Endothelial dysfunction is the pathophysiologic principle involved in the initiation and progression of arteriosclerosis, thus endothelial function serves as a "barometer" for cardiovascular health that can be used for the evaluation of new therapeutic strategies. This review provides an introduction to the concept of endothelial dysfunction, and it explores the importance of this prognostic marker in the context of clinical, dietary interventions in humans. Moreover, we summarize and evaluate the findings of various clinical trials that demonstrated an improvement of endothelial dysfunction in subjects with cardiovascular risk factors after the acute and chronic consumption of flavanol-rich foods, including cocoa products, red wine, and tea.