This book focuses on present knowledge of the biologic - in contrast to the psychologic - aspects of drug addiction and addictive behavior. It brings forth new lines of research in a subject that traditionally has been regarded as the domain of psychology and sociology. The individual predisposition to use addictive drugs repeatedly, and thus become addicted, varies strikingly from person to person. This implies important biological or even genetic differences, perhaps related to differences in structure and function of the reward systems of the brain. These and other biological, chemical, and genetic aspects are explored.