Interest in the human mind and its processes can be traced back to ancient Greece. In 387 BCE, Plato suggested that the brain was the seat of the mental processes. By 1637, Descartes had developed this idea and offered that humans are born with innate ideas. He went on to propose the theory of Mind-Body Dualism (i.e., that the mind and the body are two separate entities). Major debates followed regarding whether human thought was solely experiential or included innate knowledge. However, a consensus was difficult to reach given that consciousness, thoughts, ideas and attention are unobservable, and therefore difficult to scientifically evaluate. This chapter explores some of the key theories and debates within cognitive psychology and how these map onto explanations of criminal behaviour.