The blood-brain barrier (BBB) is present on three sites: the brain vessels, the choroid plexus and the arachnoid membrane. It is made of non-fenestrated endothelial or epithelial cells interconnected by tight junctions. Biochemically, the BBB is formed by bimolecular layers of phospholipids into which globular proteins are inserted. Exchanges through the BBB depend on whether the substances exchanged are soluble in water or in lipids. Hydrosoluble substances are largely excluded from the brain by the BBB; liposoluble substances pass the barrier easily by passive or active mechanisms. Water movements through the BBB are related to variations in osmolality. These data are of the utmost importance in all brain diseases with rupture of the BBB and in all neuroradiological procedures using contrast media.