This chapter examines what critical thinking is. Critical thinking actively and carefully evaluates the reasoning and evidence behind knowledge and arguments. In relation to approaches to knowledge, at an early stage humans seek ‘correct’ knowledge, at an intermediate stage they reject the ability to rank knowledge, and at a higher stage they evaluate knowledge to search for the best within the parameters available. Approaches to arguments see individuals as either followers who believe any arguments they hear, cynics who reject all arguments and evidence, and healthy sceptics who look to evaluate arguments based on underlying reasoning and evidence. Humans can be persuaded by pathos (appeals to emotion), ethos (the credibility of the source or speaker), or logos (the logic of the underlying argument). The chapter then looks at the revised taxonomy of cognitive processes and considers the concept of cognitive biases. As distinct from shallow or cynical thinkers, critical thinkers have a high-stage approach to knowledge; are healthy sceptics in relation to arguments looking to the underlying reasoning and evidence; are persuaded mostly by the logos of an argument; use the full spectrum of cognitive processes (especially higher-order); and try to avoid their own cognitive biases influencing their thinking.