Knowledge of the blood-brain barrier forms an essential component in the understanding of a large proportion of medical disciplines. Nevertheless, ignorance in this field still impedes progress in many areas, including cerebrovascular disease, Alzheimer's disease, cerebral AIDS, brain tumors, and the development of effective new neuropharmaceuticals. This introduction for both researchers and clinicians across the medical sciences is intended to aid both those beginning work directly in this area and those wishing simply to be better informed when interpreting information where the blood-brain barrier may be involved. Recent advances in both methodology and biology are detailed in fifty state-of-the-art chapters written by international authorities.