The existence of a permeability barrier between the blood and the brain has been known for more than a century; however, during the past two decades there has been a tremendous expansion in knowledge of the multiple functions of the blood-brain barrier (BBB). Besides forming a permeability barrier that limits the movement of some solutes between blood and brain, the BBB is now seen as a metabolically active tissue that facilitates and controls the brain uptake of certain solutes while helping to maintain homeostasis within the central nervous system (CNS). Current studies are stimulated not only by an interest in understanding the normal physiology of the barrier and the role that it plays in disease processes, but also by a desire to be able to manipulate barrier permeability and thereby control the delivery of pharmaceuticals to the brain.

This chapter serves as an overview of the functions of the BBB. For a more detailed discussion, the interested reader is referred to the other chapters in this monograph and to other sources in the literature (Betz et al. 1989; Bradbury 1979; Fenstermacher 1984; Fishman 1980; Goldstein and Betz 1986; Oldendorf 1982; Pardridge 1983; Rapoport 1976; Spector and Johanson 1989).