Reactive oxygen species (ROS) have been traditionally regarded as toxic byproducts of aerobic metabolism. However, ROS can also act as intracellular signaling molecules in vascular cells. ROS can mediate phenotypes in vascular endothelial and smooth muscle cells that may be considered both physiological and pathophysiological. Among these are growth, apoptosis, and survival. The specific response elicited by reactive oxygen intermediaries is determined by their specific intracellular target(s). This, in turn, is dependent on the species of oxidant(s) produced, the source and therefore subcellular localization of the oxidant(s), the kinetics of production, and the quantities produced. A fuller understanding of how ROS regulate mitogenesis and apoptosis in vascular smooth muscle and endothelial cells will permit the development of novel strategies to modify or prevent vascular diseases in which these phenotypes predominate.