It is the purpose of this book to try to offer a modern perspective on the importance of vitamins in human biology and medicine. There have been recent revivals of attempts to use vitamins as therapeutic agents by administering doses many times the recommended allowance. This use or 'megavitamin therapy' is reviewed by Dr. Reynold Spector, with particular emphasis on vitamin entry to the central nervous system. Of related interest is the chapter by Dr. John Blass discussing central nervous system manifestations of thiamin deficiency, and also the contribution by Ms. Fiona Cumming and clinical toxicity of vitamin supplementation.