Progress is being made in identifying time-dependent, drug- induced adaptations and relating them to specific behavioral features of drug addiction. Current research needs to clarify the types of adaptations associated with particularly long-lived aspects of drug addiction, such as drug craving and relapse, and to identify specific genes that contribute to individual differences in vulnerability to drug addiction. Understanding the molecular and cellular basis of addictive states will lead to major changes in how drug addiction is viewed and ultimately treated. A biological understanding of drug addiction requires knowledge of how acute drug effects on the brain are transformed into progressively longer-lasting adaptations in specific brain regions, and further research in this area is recommended. 63 references and 3 figures