Reactive oxygen species (ROS), byproducts of oxygen metabolism, are present in the cells as a consequence of living in an oxygen-rich atmosphere. ROS can be generated by both endogenous and exogenous sources, such as mitochondria and carcinogens, respectively. ROS contain superoxide (O2•−) and hydrogen peroxide (H2O2), and are important for the normal function of many cellular processes, including metabolism, cell growth and differentiation, immune responses and apoptosis. Low levels of ROS serve as secondary messengers and are essential for carrying out these cellular functions. Overproduction of ROS and generation of highly reactive ROS, for example hydroxyl (•OH) radicals, can attack lipids, protein, DNA, and other cellular components, leading to numerous diseases, among them cancer, and cardiovascular and neurological disorders. At the Graduate Center for Toxicology, investigators specializing in such fields as redox signaling, mitochondria, regulation of free radical-mediated proteins, lipid and DNA damage pursue multidisciplinary approaches to translate mechanistic insights into measures to predict, prevent, and treat diseases. Students admitted to the Graduate Center for Toxicology will take classes in subjects focused on these fields and will engage in research led by professors who are leaders in their fields.