Neuroimaging studies have been crucial in understanding changes in the various neurotransmitter systems implicated in addiction in the living human brain. Predominantly reduced striatal dopamine transmission appears to play an important role in psychostimulant, alcohol and heroin addiction, while addiction to cannabis may be mediated primarily by the endocannabinoid system. However, the study of other neurotransmitter systems likely involved in addiction, for example glutamate, has been limited by the number and quality of available radiotracers, and data on changes in these systems in the most common addictions are emerging only now. Further studies are needed to understand fully how the interplay of various neurotransmitter systems contributes to addiction and to ultimately help to develop more effective treatment approaches.