I argue that denial plays a central but insufficiently recognized role in addiction. The puzzle inherent in addiction is why drug use persists despite negative consequences. The orthodox conception of addiction resolves this puzzle by appeal to compulsion; but there is increasing evidence that addicts are not compelled to use but retain choice and control over their consumption in many circumstances. Denial offers an alternative explanation: there is no puzzle as to why drug use persists despite negative consequences if these consequences are not straightforwardly known. I describe the nature of the causal knowledge that one's drug use is causing negative consequences; map the conceptual landscape of denial and explain how it can block such knowledge; and explore some of the processes and mechanisms that have been studied by philosophy and the cognitive sciences and which may underpin denial in addiction, including well-established information-processing biases, motivational influences on belief formation and self-deception, and cognitive deficits with respect to insight and self-awareness. I conclude by suggesting that addiction is as much a disorder of cognition as a disorder of conation.