 We use the term addiction to cover a whole range of different behavioural problems that people experience in controlling behaviours. Many years ago, of course, the term was used quite narrowly to cover problems that people had with what we would now consider to be physical addiction. You take a drug, you get used to the drug, your body adapts physiologically and then when you stop taking the drug you suffer unpleasant or even life threatening withdrawal symptoms. We no longer use the term in that way for a very good reason and that is we've come to understand that there's now, there's actually a wide range of different kinds of behaviours which fall into this broad category that we want to call addiction because the real problem is not the withdrawal symptoms per se but it's what happens to the person's free will if you like, what happens to the choices that they make, the behaviours that they engage in. We now recognise that addiction covers things that don't even involve drugs, gambling for example, some forms of internet use and even issues such as eating disorders or obesity. So there is this general term that we're now using to describe a behavioural problem in which there's a powerful motivation to engage in a behaviour which has been learnt through experience and which is causing harm. The way that society treats them affects the problems that the user's experience. So for example when you have something like alcohol dependence or addiction to cigarettes, both of which comprise behaviours which are essentially legal, then you've got a different set of problems than if you're trying to deal with problems associated with illicit drug use. The fundamental differences arise from the social context, the pharmacology of the drug if there is a drug involved and the psychological reactions to the drug and the social context in which it occurs. The way we understand addiction affects what we do to try to combat it. Let's say you're a politician or working in an agency that's involved in interventions to try to combat addiction. If your model of addiction is that it is fundamentally about choice, about free choice that addicts make, then you will engage in a certain kind of intervention, you will engage in interventions that affect that choice. For example you'll use the criminal justice system, you'll use taxation, you'll use mass media campaigns to try to tackle the problem. If you're someone who believes that addiction is fundamentally a brain disease, then it's a different set of interventions, you will try to look for better drugs to help people to stop or better behavioural support programmes to treat this disease process. So it makes a very big difference how you view the problem. The reason for writing a book on models of addiction is that everyone, whether it's the lay, public, politicians, researchers, clinicians, all have an idea about what they understand by addiction and there's about as many ideas as there are people. And really what we need to do is to bring all these together into a more comprehensive and coherent framework so that we can actually do a much better job of tackling the problem.