Severe head injury often produces complex intracranial displacements of the brain, resulting in widespread, often microscopic lesions. These are responsible for two types of edema: vasogenic edema, with outflow of molecules and fluid into the extracellular spaces by rupture of the blood-brain barrier and vasoplegia, and cytotoxic edema, with swelling of astrocytes due to membrane lesions. The connexions between these two types of edema are still obscure. Alterations in membrane phospholipids may impede function of Na-K pump enzymes, causing accumulation of water in the cell. Cerebral edema is responsible for intracranial hypertension and tentorial herniation, which in turn increase edema through venous compression, ischemia, and hypoxia. The least controversial anti-edema therapeutic measures include relative fluid and salt restriction, mannitol if called for, neuroplegia, in particular with diazepam and Gamma-OH, and assisted ventilation.