Dopamine neurons in the dopamine system that originate in the ventral tegmental area and project to the nucleus accumbens, olfactory tubercle and frontal cortex have long been implicated in reward and motivational processes. The accumulation of substantial neuropharmacological data and recent data from electrophysiological recording studies in primates and recent theoretical modelling studies provide new significant insights into the function of these mesolimbic dopamine neurons that have important implications for psychiatry. Appetitive events and not aversive events activated dopamine neurons in the mesolimbic dopamine system of primates. Modelling studies show that dopamine neurons may be responsible for initiating action associated with significant changes in the value of incentives in the environment and neuropharmacological data show that the activation associated with approach to incentives is abolished by removal to the mesolimbic dopamine system. Based on these three lines of research it is hypothesized that the function of the mesolimbic dopamine system is to allow or actually release species-specific approach responses or modifications in direction toward changes in positive incentives. These results have implications not only for our understanding of the role of dopamine in mental disorders, but also for our understanding of the role of dopamine in specific aspects of drug dependence.