Brain edema is a life-threatening complication of cerebral infarction. The molecular cascade initiated by cerebral ischemia includes the loss of membrane ionic pumps and cell swelling. Secondary formation of free radicals and proteases disrupts brain-cell membranes, causing irreversible damage. New diagnostic methods based on magnetic resonance imaging have markedly improved diagnostic accuracy. Cytotoxic and vasogenic edema is maximal by 24 to 72 hours after the ischemic event. Thrombolytics reperfuse tissue and improve outcome; when treatment is delayed, they can increase edema and blood-brain barrier opening. Although osmotherapy reduces brain water, and is used to treat ischemic edema, its efficacy remains to be proven. As the molecular events become clearer, novel treatments that block different stages of the injury cascade will be available for clinical testing.