Association of ideas, or mental association, is a process by which representations arise in consciousness, and also for a principle put forward by an important historical school of thinkers to account generally for the succession of mental phenomena. The term is now used mostly in the history of philosophy and of psychology. One idea was thought to follow another in consciousness if it were associated by some principle. The three commonly asserted principles of association were similarity, contiguity, and contrast, numerous others had been added by the nineteenth century. By the end of the nineteenth century physiological psychology was so altering the approach to this subject that much of the older associationist theory was rejected.

Everyday observation of the association of one idea or memory with another gives a face validity to the notion. In addition, the notion of association between ideas and behavior gave some early impetus to behaviorist thinking. The core ideas of associationist thinking recur in some recent thought on cognition, especially consciousness.