The aim of this article is to argue that the ancient concept of akrasia can serve as the conceptual foundation and theoretical justification for a dual systems theory of addiction. Akrasia refers to acting against one's better judgement and thus to behaviour that agents know is bad for them and have previously resolved to abjure, addiction being seen as an extreme form of akrasia. In this way akrasia can provide the basis for accounts of addictive behaviour that stress its inconsistency over time and the great difficulty addicts experience in changing it. A definition of addiction consistent with this view is offered and defended, and some of its general advantages for understanding and responding to addiction described. A consequence of defining addiction in this way is that it should be seen as a disorder of choice or, alternatively, of self-regulation, and reasons are given why the latter term should be preferred. It is then proposed that the concept of akrasia and the perspective on addiction that follows from it lead logically to the need for a dual systems theory of addiction. Although no such theory is presented here, terminological issues are clarified, criticisms of dual systems theories are noted, and a rudimentary description of what an integrated dual systems theory of addiction might look like is provided.