This review outlines the basic concepts and pathophysiological aspects of the chemistry of free radicals in all forms of brain injury and brain edema. Brain edema, defined as an abnormal accumulation of fluid associated with volumetric enlargement of the brain, is complicated and multifactorial, and it is still a serious complication of strokes, brain tumors, brain abscesses and hypoxic injury. Recent experimental studies have demonstrated that free radicals play an important role as chemical mediators in causing brain edema and brain damage. Pharmacological antagonism of free radicals with free radical scavengers shows beneficial therapeutic results. Future research into brain edema and optimal combinations of therapeutic regimens is expected to improve the management of patients with brain edema.