Developing technology in the field of neuroscience has revolutionized the scientific understanding of the brain. The field of addiction, in particular, has benefited greatly from an enhanced understanding of the mechanisms of drug action, and the changes in brain function that result from chronic drug use. An interesting, and parallel field of research that has also seen an exponential gain in knowledge, has been the study of the biological basis of impulsivity. Indeed, there is now strong evidence that the neural mechanisms implicated in addiction also play a prominent role in impulsive behaviour more generally. This has led to a reconsideration of the role of personality in both the initiation of substance use, and the development of problem use. In this talk we will provide an overview of Reinforcement Sensitivity Theory (J.A. Gray) and argue that this biologically-based model of personality has particular relevance for the study of addictive behaviours. We will focus on the role of impulsivity, arguing that it is more accurately conceptualized as two separate dimensions. The first, Reward Drive, reflects an increased sensitivity to reward and is related to the functioning of the mesolimbic dopamine system. The second trait, Rash Impulsiveness, reflects individual differences in the ability to modify or inhibit behavior in light of potential negative consequences. At the neurophysiological level, this trait is related to the functioning of the orbitofrontal and anterior cingulate corticies, as well as their associated connections with the mesolimbic system. The papers that follow provide evidence for the role of impulsivity in addictive behaviors.