A free radical is a highly reactive chemical species that can react with organic macromolecules leading to cell and tissue damage and consequent functional disruption. Free radical or oxidative injury is increasingly recognized as an important factor in the pathophysiology of many human diseases, including those that affect the nervous system. This review summarizes important evidence implicating oxidative injury in the pathogenesis and progression of many important neurological disorders, including cerebrovascular disease, epilepsy, amyotrophic lateral sclerosis, and Huntington's disease. Results of controlled clinical trials of various antioxidant therapies in neurological disease performed to date are also highlighted.