INTRODUCTION The central hypothesis guiding our research during the past decade or more has been that drug addiction can be understood in terms of the operation of the brain’s learning and memory systems (Robbins & Everitt 1999; Everitt et al. 2001; Everitt & Robbins 2005). In particular, that chronically self-administered drugs may in some way pathologically subvert these memory systems and so lead to the establishment of compulsive drug-seeking habits (Everitt & Robbins 2005). Initially, our approach to the understanding of drug addiction built upon the clinical insight embodied within the DSM-IV (1994) criteria for ‘substance abuse’ and ‘substance dependence’. Therefore, we began experimentally to decompose and augment the DSM-IV diagnostic framework in terms of specified learning and cognitive processes deriving from animal learning theory and that increasingly have been attributed to the operation of specific neural, especially limbic cortical–striatal, systems (Everitt et al. 2001). The early focus of much experimental drug addiction research was to understand the reinforcing, or *Author for correspondence (bje10@cam.ac.uk). Electronic supplementary material is available at http://dx.doi.org/10. 1098/rstb.2008.0089 or via http://journals.royalsociety.org. One contribution of 17 to a Discussion Meeting Issue ‘The neurobiology of addiction: new vistas’. ‘rewarding’, effects of abused drugs; this has led to great advances in defining the primary molecular targets of addictive drugs as well as, more recently, the adaptations in these targets that develop with chronic drug self-administration (Nestler 2004; Koob & Le Moal 2005). However, it has been appreciated for some time that the molecular and neurochemical correlates of acute and chronic drug administration must be interpreted in behavioural and cognitive terms if the psychological processes and neurobiological mechanisms determining human drug addiction are to be specified. Therefore, we and others have increasingly come to view drug addiction as the endpoint of a series of transitions from initial drug use—when a drug is voluntarily taken because it has reinforcing, often hedonic, effects—through the loss of control over this behaviour, such that it becomes habitual and ultimately compulsive. We have recently reviewed the evidence that these transitions depend upon interactions between Pavlovian and instrumental learning processes (Everitt & Robbins 2005). Furthermore, we have hypothesized that the ‘switch’ from voluntary drug use to habitual and progressively compulsive drug use represents a transition at the neural level from prefrontal cortical to striatal control over drug-seeking and drug-taking behaviours, as well as a progression from ventral to more dorsal domains of the striatum, mediated at least in part by its stratified dopaminergic innervation (Everitt & Robbins 2005).