Each year in the United States, the use of tobacco claims an estimated 440,000 lives while the use of illicit drugs and alcohol claims another 100,000 lives. The science of addiction is explained, which is defined as a chronic, relapsing brain disease characterized by compulsive drug seeking and use despite its harmful consequences. The harmful consequences of drug abuse are varied and can include adverse medical, social, economic, and criminal justice outcomes. Drug addiction is considered a brain disease because the substances actually change the structure and operation of the brain. The main reasons people take drugs are reviewed followed by a description of brain imaging photographs of drug-addicted individuals, which show the physical changes drugs can have on the brain in areas critical to decisionmaking, learning, memory, and impulse control. Many scientists believe that these changes help explain the compulsive and destructive behaviors of addiction. The mechanisms of addiction are explained in more depth, which account for why some people will become addicted while others will not. The environmental and other factors that increase the risk of drug addiction are identified followed by a discussion on the best strategies for preventing drug abuse, particularly among children and adolescents. Three types of strategies are highlighted: (1) universal programs that focus on the risk and protective factors common to most children in most settings; (2) selective programs that target groups on children and adolescents most at risk for increasing their drug abuse; and (3) indicated programs that are designed for youths who have already begun abusing drugs. The medical consequences of drug abuse are reviewed followed by a consideration of effective addiction treatments, which indicate that addiction is a treatable disease. Exhibits