The blood-brain barrier serves to protect the brain from toxic substances whilst simultaneously allowing access to essential nutrients and chemical signals. 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 of both the biology of this important membrane and the methodology suitable for its investigation still remains an impediment to progress in many fields, including, for example, the development of new and efficacious neuropharmaceuticals, cerebrovascular disease, Alzheimer's disease, cerebral AIDS and brain tumours. 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. Advances in both methodology and biology are detailed in 50 chapters from international authorities.