Reactive oxygen and nitrogen species are thought to contribute to pathogenesis of many cardiovascular diseases including hypertension, atherosclerosis, restenosis, heart failure, and diabetic vascular complications. Some of these reactive oxygen species also play an important role in vascular signaling. In this chapter, we describe various techniques that we have successfully employed to reliably measure superoxide and hydrogen peroxide. Because reactive oxygen species are capable of rapidly inactivating nitric oxide and because endothelial function characterized by nitric oxide bioavailability is an important indicator of vascular health, we have also included novel techniques capable of directly measuring nitric oxide radical from vascular cells and tissues.