 Molecular oxygen, O2, is the primary biological electron acceptor that plays essential roles in fundamental cellular processes. However, its beneficial properties also come at the cost of the formation of reactive oxygen species, ROS, like superoxide, O2, hydrogen peroxide, and hydroxyl radical, OH. These ROS can be detrimental to aerobic cells if left unchecked, so organisms have developed both non-enzymatic and enzymatic antioxidant defenses. Non-enzymatic antioxidants include catalases, peroxidases, superoxide dismutases, and glutathione S transferases, GSTs. Enzymatic antioxidants include those that break down superoxide into molecular oxygen and hydrogen peroxide, such as catalases and superoxide dismutases. Additionally, cellular ROS sensors are not fully understood, but a few transcription factors that regulate the expression of antioxidant genes have been identified in prokaryotes and yeasts. In higher eukaryotes, oxidative stress responses are more complex. This article was authored by J.G. Scandalios. We're article.tv, links in the description below.