This book describes what molecular imaging is, how it developed, what are its basic principles, and what it has told us and can tell us about the chemistry of the human brain. Everyone today is conscious of the fact that there is chemistry going on in the brain, and that it is affected by widely used pharmaceuticals and illicit drugs. This book will elucidate these topics in an interesting, historical and philosophical way. The book is a valuable reference resource for all those in nuclear medicine and radiology as well the educated general public.