Cerebral edema is a common clinical disorder that results from an abnormal increase in water content within the extracellular (EC) compartment of the brain. It is distinguished from two other types of brain bulk enlargement: (1) vascular swelling, caused by arterial dilatation or venous obstruction; and (2) cellular swelling, caused by cytotoxic injuries or metabolic storage. Under normal conditions, the EC compartment has two fluids, the interstitial fluid (ISF) and the cerebrospinal fluid (CSF), and extends from the blood brain barrier (BBB) through a series of 100 to 150-A-wide intercellular spaces that are anatomically continuous with the CSF spaces. There are four primary types of EC edema: (1) vasogenic edema, which results from an increase in brain capillary permeability, the most common type, in which leakage of plasma constituents into the brain follows the pathways of ISF bulk flow and is governed by the interaction of systemic arterial pressure and tissue resistance; (2) osmotic edema, which results from an unfavorable osmotic gradient between the plasma and ISF across an intact BBB; (3) compressive edema, which results from obstruction of ISF bulk flow pathways; and (4) hydrocephalic edema, which results from obstruction of CSF bulk flow pathways. In this latter type of edema, distension of the collecting channels proximal to the block leads to retrograde flooding of the EC compartment with the formation of periventricular edema. The syndrome of pseudotumor cerebri includes several different types of brain bulk enlargement.