Endothelial activation refers to a specific change in endothelial phenotype, characterized most notably by an increase in endothelial-leukocyte interactions and permeability, which is pivotal to inflammatory responses in both physiologic and pathologic settings. An increasing body of evidence indicates an important role for reactive oxygen species (ROS)-mediated modulation of signal-transduction pathways in many of the processes involved in endothelial activation. ROS generated by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family of enzymes may be especially important in this regard. We discuss the evidence implicating redox signaling pathways in the molecular and cellular processes underlying endothelial activation and the role in cardiovascular diseases, and also provide a detailed description of NADPH oxidase regulation in endothelial cells, in view of its likely importance in this context.