This booklet explores various aspects of drug addiction, with a special focus on drugs' effects on the brain. A brief introduction presents information on the rampant use of drugs in society and elaborates the distinction between drug abuse and drug addiction. Next, a detailed analysis of the brain and its functions is given. Drugs target the more primitive portions of the brain, an action which allows them to override the cognitive processes of higher brain function. Explained is how pleasure acts as a powerful biological force to ensure survival, and how drugs act on the brain's pleasure cells. Drug addiction, it is argued, is a biologically based disease that actually alters the brain's pleasure networks. Discussed are how drugs interact with nerve cells in the brain and how drugs change the normal process of chemical neurotransmission. Finally, some of the special effects of different classes of drugs and the particular brain areas they target are examined. These include opiates, cocaine, marijuana, hallucinogens, PCP, depressants, and designer drugs. Other aspects of drug addiction are presented, such as the steps to addictions, the fetus and addiction, treatment, and genetics. (RJM)