 Molecular oxygen is a vital biological electron acceptor that forms reactive oxygen species, which can harm or kill aerobic cells. To minimize these harmful effects, organisms evolved antioxidant defenses such as catalases, peroxidases, superoxide dismutases, and glutathione S-transferases. These antioxidants are regulated by transcription factors in mammals, including nuclear factor Kb and activator protein 1, while plants have their own specific transcription factors such as the antioxidant responsive element. ROS have been implicated in various environmental stress conditions that activate plant defense genes. This article was authored by J.G. Scandalios.