 Molecular oxygen is a vital biological electron acceptor that forms reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radical, which can harm or kill aerobic cells. To minimize these damaging effects, aerobic organisms evolved antioxidant defenses including enzymes like catalysis, peroxidases, superoxide dismutases, and glutathione S-transferases. These antioxidant mechanisms are regulated by transcription factors such as nuclear factor KB and activator protein H1 in mammals, and the antioxidant responsive element in plants. ROS have been implicated in environmental stress conditions that activate plant defense genes like catalysis and superoxide dismutases. This article was authored by JG Scandalios.