The complexity of drug addiction mirrors the complexity of the psychological processes that motivate animals to work for any reinforcer, be it a natural reward or a drug. Here, we review the role of the nucleus accumbens, together with its dopaminergic and cortical innervation, in responding to reinforcement. One important contribution made by the nucleus accumbens is to the process through which neutral stimuli, once paired with a reinforcer such as a drug, have the capacity to motivate behaviour. This process may be one of several contributing to addiction, and it may be amenable to pharmacological intervention.