What is cognitive psychology? The current article answers this question by describing cognitive psychology’s core ideas, how these ideas entered into experimental psychology, and how these ideas have evolved over time. Cognitive psychology’s core ideas lead to its information processing hypothesis: the claim that human cognition is the rule-governed manipulation of mental representations. The article describes the context in which basic ideas about information processing inspired experimental psychologists who adopted and adapted them to create a new school of psychology. The article describes how the ideas permitted cognitive psychology to replace an earlier school, behaviorism. The article considers cognitive psychology as a constantly evolving school of thought and discusses differences between early cognitive psychology and its modern form.