Addiction to opiates and to alcohol is a major cost to the public both in terms of ill-health and in crime and policing. Intoxication with these drugs has huge costs to society in terms of accidents and violence. Together these costs have been estimated as equivalent to those of all other psychiatric disorders combined (ca. #4billion p.a. in the UK). Understanding the actions of these drugs in the brain is a necessary component in combating these problems. We are trying to identify the brain mechanisms of addiction to, and pleasure from, these drugs. We know that drugs including alcohol act by changing chemical messengers in the brain called neurotransmitters. Our study tries to show how these are altered in people addicted to heroin or alcohol. We also want to understand how treatments work, so we also study the effects that current treatments have in the brain to help design new and better ones. This could have big implications for health and reduce the costs of the damage that these drugs do to society. It may also lead to new investment from pharmaceutical companies which are one of the UKs leading wealth-creating industries. This work continues our successful previous research which uses the newest scanning technique, positron emission tomography (PET). This allows us to directly measure levels of neurotransmitters and their receptors in the human brain. We also perform studies using drugs to alter these neurotransmitter systems to investigate how they are functioning in healthy individuals and those that are addicted. We focus on three key neurotransmitters in addiction- the dopamine system involved in mediating pleasure, the GABA system, the brains natural calming system that mediates many effects of alcohol, and lastly the opiate system which mediates many effects of opiate drugs such as heroin and also the pleasurable effects of alcohol.