Extraordinary Popular Delusions and the Madness of Crowds is an early study of crowd psychology by Scottish journalist Charles Mackay, first published in 1841 under the title Memoirs of Extraordinary Popular Delusions. The book was published in three volumes: "National Delusions", "Peculiar Follies", and "Philosophical Delusions". Mackay was an accomplished teller of stories, though he wrote in a journalistic and somewhat sensational style.

Crowd psychology is a branch of social psychology that deals with the ways in which the psychology of a crowd is different from the psychology of the individual persons who are in the crowd. The field of crowd psychology enquires into the behaviors and thought processes of both the individual members of the crowd and the crowd as a collective social entity. The behavior of a crowd is much influenced by deindividuation, a person's loss of responsibility, and the person's impression of the universality of behavior, both of which conditions increase in magnitude with size of the crowd. Notable theorists in crowd psychology include Gustave Le Bon, Gabriel Tarde, and Sigmund Freud.