Current understanding of drug abuse has been greatly influenced by the emphasis on drug-seeking behaviour as the common element. The three main attributes of drugs that maintain, direct and regulate drug-seeking behaviour are their positively reinforcing and discriminative and aversive stimulus properties. Each process may be analysed in terms of underlying behavioural and neural mechanisms that are mutually complementary and interactive. Environmental stimuli conditioned to the effects of the drugs also play a key role in eliciting and maintaining drug-seeking behaviour. Both the behavioural and the neural mechanisms are subject to modulating variables such as social, environmental and genetic factors, including the previous behavioural and pharmacological history of the individual. Thus, the behavioural approach to addiction does not preclude important roles for other factors, but rather seeks to integrate them into a comprehensive theoretical framework strongly linked to empirical data.