Reactive oxygen species (ROS) have been shown to play important functions as signaling molecules in the vasculature. Several NADPH oxidase isoforms have been identified in endothelial and smooth muscle cells, which are largely responsible for basal and stimulated ROS production in these cells. Moreover, evidence increases that activation of these enzymes contributes to stimulation of signaling cascades modulating essential functions in the vasculature such as endothelial and smooth muscle cell proliferation as well as thrombosis.