Vasogenic edema is due to a breakdown of the blood-brain barrier. This is the most common type of brain edema encountered in clinical medicine. In vasogenic edema, CT shows hypodense frond-like regions located within the white matter. This edema surrounds the underlying pathological lesion. The edema may be quite extensive in relation to the size of the lesion. Cytotoxic edema is due to impaired brain metabolism with swelling of neurons, glial, and endothelial cells. In cytotoxic edema, CT shows diffuse hypodense subcortical regions. Ischemic edema is due to impaired tissue perfusion. The CT appearance is that of a hypodense region with a specific arterial vascular distribution. Interstitial edema is due to hydrocephalus, CT shows hypodense regions directly capping the ventricles or the mass lesion. Based upon the CT findings, it is frequently possible to determine the type of edema present and to predict with greater accuracy the nature of the underlying pathological process.