At the interface between brain and body, knowledge of the blood-brain barrier forms an essential component in the complete understanding of a large proportion of medical disciplines. Nevertheless, it seems that ignorance still remains an impediment to progress in many fields, including amongst others the development of new and efficacious neuropharmaceuticals, cerebrovascular disease, Alzheimer's disease, cerebral AIDS and brain tumors. 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 50 state-of-the-art chapters from international authorities.