Oxygen is essential to aerobic life, but it is also associated with the production of highly reactive compounds that can pose danger to physiologic systems when the oxygen concentration is excessive. Reactive oxygen species (ROS) are required for normal physiologic processes, but when produced in excess, they can overwhelm endogenous antioxidants, resulting in significant cellular damage and, eventually, cell death. Ischemic events can initiate numerous pathophysiologic mechanisms leading to increased production of ROS, loss of cellular energy production, and lipid peroxidation. Although reperfusion is a necessary step in cellular recovery from ischemia, it can be deleterious by leading to the generation of even more ROS and stimulating the accumulation of neutrophils. Both of these processes may contribute to irreversible cell death and, ultimately, organ failure. This article reviews oxygen metabolism, ischemia, and reperfusion injury and how these processes may occur in equine disorders.