More and more diseases have been proposed to have a radical or oxidant involvement. Although in most cases we do not know if this involvement is a cause or a result of the disease process, it is still valuable to learn about those compounds or enzymes that might block, inhibit, or prevent radical-initiated reactions. Therefore, it becomes increasingly important to understand which compounds can function as antioxidants, where they are located in the body, and what their mechanism of action might be. As we increase our knowledge in these areas, we will have a better opportunity to propose interventions that might suppress or even reverse some of the ravages of oxidant-based diseases in humans.