The term "blood-brain barrier" was coined over one hundred years ago as a result of the observation that vital dyes introduced into the circulation quickly penetrated and stained nearly all organs and tissues of the mammalian body except the brain which retained its pale creamy appearance. Advances in microscopy revealed that, in contrast to other vascular beds, the brain endothelial cells lining the vascular wall are tightly linked with junctional complexes that eliminate gaps or spaces between cells and prevent any free diffusion of blood-borne substances into the brain parenchymal space. The endothelial cells, situated at the interface between blood and brain, therefore, play a critical role in performing essential biological functions including transport of micro- and macronutrients, receptor-mediated signaling, leukocyte trafficking, and osmoregulation. A number of molecular components responsible for some of these unique properties have now been identified and are being characterized under physiological and disease conditions. These include the proteins involved in formation and assembly of tight junctions; the plasma membrane-embedded proteins that are responsible for transport of brain energy substrates and nutrients (glucose, monocarboxylic acids, nucleosides, amino acids, others); the multi-drug transporter protein, p-glycoprotein, and other drug-rejecting proteins that protect the brain from foreign, potentially disruptive chemicals. These and other recent findings, taken as a whole, reveal the brain endothelium as a complex and dynamic biological system, in contrast to the simple, inert and rigid barrier initially perceived.