Persistent changes in behavior and psychological function that occur as a function of experience, such those associated with learning and memory, are thought to be due to the reorganization of synaptic connections (structural plasticity) in relevant brain circuits. Some of the most compelling examples of experience-dependent changes in behavior and psychological function, changes that can last a lifetime, are those that accrue with the development of addictions. However, until recently, there has been almost no research on whether potentially addictive drugs produce forms of structural plasticity similar to those associated with other forms of experience-dependent plasticity. In this paper we summarize evidence that, indeed, exposure to amphetamine, cocaine, nicotine or morphine produces persistent changes in the structure of dendrites and dendritic spines on cells in brain regions involved in incentive motivation and reward (such as the nucleus accumbens), and judgment and the inhibitory control of behavior (such as the prefrontal cortex). It is suggested that structural plasticity associated with exposure to drugs of abuse reflects a reorganization of patterns of synaptic connectivity in these neural systems, a reorganization that alters their operation, thus contributing to some of the persistent sequela associated with drug use--including addiction.