 Oxygen is converted into reactive oxygen species, ROS, which can cause damage to cells and tissues when present at high levels. However, low to moderate levels of ROS play important roles in biological processes, such as protein phosphorylation, ion channel regulation, and cross-linking of extracellular matrix. Additionally, ROS are necessary for proper functioning of the immune system, thyroid hormones, and other metabolic pathways. When ROS levels become excessive, this can lead to various diseases, such as chronic granulomatous disease, certain autoimmune disorders, and cardiovascular and neurodegenerative diseases. Antioxidants are not effective in treating these conditions, likely because they are unable to reach the site of injury before the damage is done. Instead, targeted inhibition of ROS-producing enzymes is a more promising approach for treating these diseases. This article is authored by Catherine Breaker, Stefania Shavoni, Francis J. Miller Jr., and others.