Current theories propose that reasoning comprises two underlying systems (Evans & Over, 1996; Sloman, 1996; Stanovich & West, 2000). The systems are identified as having functionally distinct roles, differ according to the type of information encoded, vary according to the level of expressible knowledge, and result in different responses. This article evaluates the arguments and the evidence from a select number of key tasks that have been supportive of dual-reasoning theorists' proposals. The review contrasts the dualist approach with a single-system framework that conjectures that different types of reasoning arise through the graded properties of the representations that are utilized while reasoning, and the different functional roles that consciousness has in cognition. The article concludes by arguing in favor of the alternative framework, which attempts to unify thedifferent forms of reasoning identified by dual-process theorists under a single system.