This review article outlines basic concepts and pathophysiological aspects of the chemistry of oxygen free radicals in all forms of brain injury and brain edema. Recent experimental studies have demonstrated that oxygen free radicals may be important mediators of brain injury and brain edema, and pharmacological antagonism of oxygen free radicals shows beneficial therapeutic results. A number of fundamental questions need to resolved, and advanced techniques for detecting oxygen free radicals will be needed. No clinical data are available, but oxygen free radical scavengers may possibly become a critical therapeutic modality for brain injury and brain edema.