Drug addiction, like all psychiatric disorders, is defined solely in behavioural terms. For example, addiction can be considered a loss of control over drug-taking, or compulsive drug-seeking and -taking despite horrendous consequences. Abnormal behaviours are a consequence of aberrant brain function, which means that it is a tangible goal to identify the biological underpinnings of addiction. The genetic basis of addiction encompasses two broad areas of enquiry. One of these is the identification of genetic variation in humans that partly determines susceptibility to addiction. The other is the use of animal models to investigate the role of specific genes in mediating the development of addiction. Whereas recent advances in this latter effort are heartening, a major challenge remains: to understand how the many genes implicated in rodent models interact to yield as complex a phenotype as addiction.