Human Brain microvascular endothelial cells (BMEC) is the major component of the blood-brain barrier. One of their functions is to limit the passage of soluble and cellular substances from the blood into the brain. BMEC have unique features to distinguish themselves from those of peripheral endothelial cells, such as; 1) intercellular tight junctions that display high electrical resistance and also a slow paracellular flux; 2) the absence of fenestrae and a reduced level of pinocytic activity; and 3) asymmetrically-localized enzymes and carrier-mediated transport systems. Similar to peripheral endothelial cells, BMEC can express cell adhesion molecules on their surface that regulate the extravasation of leukocytes into the brain. BMEC are useful tools for studying the molecular and cellular properties of blood-brain barrier because of their unique functions.