Endothelial cells have numerous endocrine functions and contribute to a variety of processes, including penile erection and vasodilation. Endothelial dysfunction is associated with cardiovascular risk factors and has been implicated in the pathogenesis of atherosclerosis and ED. This study reviews endothelial function, in addition to endothelial dysfunction and its role in atherosclerosis and ED. Measurement of endothelial function is reviewed, including catheter-based methods, venous occlusion plethysmography, high-frequency ultrasound, peripheral arterial tonometry, digital pulse amplitude tonometry, digital thermal monitoring, the L-arginine test and measurement of compounds released by endothelial cells. Therapy and medications that improve endothelial function are reviewed. As the scientific community learns more about the importance of the endothelium, it is increasingly important for the clinician to understand endothelial function, dysfunction, measurement of endothelial function and therapies that affect this remarkable cell type.