Addictive drugs have habit-forming actions that can be localized to a variety of brain regions. Recent advances in our understanding of the chemical 'trigger zones' in which individual drugs of abuse initiate their habit-forming actions have revealed that such disparate drugs as heroin, cocaine, nicotine, alcohol, phencyclidine, and cannabis activate common reward circuitry in the brain. Although these drugs have many actions that are distinct, their habit-forming actions (and perhaps the relevant elements of their disparate withdrawal symptoms) appear to have a common denominator, namely, similar effects in the brain mechanisms of reward.