Crowd psychology (or mob psychology) is a subfield of social psychology which examines how the psychology of a group of people differs from the psychology of any one person within the group. The study of crowd psychology looks into the actions and thought processes of both the individual members of the crowd and of the crowd as a collective social entity. The behavior of a crowd is much influenced by deindividuation (seen as a person's loss of responsibility) and by 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 (1841-1931), Gabriel Tarde (1843-1904), and Sigmund Freud (1856-1939). Many of these theories are today tested or used to simulate crowd behaviors in normal or emergency situations. One of the main focuses in these simulation works aims to prevent crowd crushes and stampedes.

According to his biological theory of criminology, which suggests that criminality is inherited and that someone "born criminal" could be identified by the way they look, Enrico Ferri expressed his view of crime as a degeneration more profound than insanity, for in most insane persons the primitive moral sense survives the wreck of their intelligence. Along similar lines were the remarks of Benedickt, Sergi and Marro. A response from the French, who put forward an environmental theory of human psychology, M. Anguilli called attention to the importance of the influence of the social environment upon crime. Professor Alexandre Lacassagne thought that the atavistic and degenerative theories as held by the Italian school were exaggerations and false interpretations of the facts, and that the important factor was the social environment."

In Paris during 10–17 August 1889, the Italian school received a stronger rebuke of their biological theories during the 2nd International Congress of Criminal Anthropology. A radical divergence in the views between the Italian and the French schools was reflected in the proceedings.

Earlier, literature on crowds and crowd behavior had appeared as early as 1841, with the publication of Charles Mackay's book Extraordinary Popular Delusions and the Madness of Crowds. The attitude towards crowds underwent an adjustment with the publication of Hippolyte Taine's six-volume The Origins of Contemporary France (1875). In particular Taine's work helped to change the opinions of his contemporaries on the actions taken by the crowds during the 1789 Revolution. Many Europeans held him in great esteem. While it is difficult to directly link his works to crowd behavior, it may be said that his thoughts stimulated further study of crowd behavior. However, it was not until the latter half of the 19th century that scientific interest in the field gained momentum. French physician and anthropologist Gustave Le Bon became its most-influential theorist.

There is limited research into the types of crowd and crowd membership and there is no consensus as to the classification of types of crowds. Two recent scholars, Momboisse (1967) and Berlonghi (1995) focused upon purpose of existence to differentiate among crowds. Momboisse developed a system of four types: casual, conventional, expressive, and aggressive. Berlonghi classified crowds as spectator, demonstrator, or escaping, to correlate to the purpose for gathering.

Another approach to classifying crowds is sociologist Herbert Blumer's system of emotional intensity. He distinguishes four types of crowds: casual, conventional, expressive, and active. A group of people who just so happen to be at the same location at the same time is known as a casual crowd. This kind of mob lacks any true identity, long-term goal, or shared connection. A group of individuals who come together for a particular reason is known as a conventional crowd. They could be going to a theater, concert, movie, or lecture. According to Erich Goode, conventional crowds behave in a very conventional and hence somewhat structured manner; as their name suggests, they do not truly act out collective behavior. A group of people who come together solely to show their excitement and feelings is known as an expressive crowd. A political candidate's rally, a religious revival, and celebrations like Mardi Gras are a few examples.  An active crowd behaves violently or in other damaging ways, such looting, going above and beyond an expressive crowd. One of the main examples of an acting crowd is a mob, which is an extremely emotional group that either commits or is prepared to do violence. A crowd changes its level of emotional intensity over time, and therefore, can be classed in any one of the four types.

Generally, researchers in crowd psychology have focused on the negative aspects of crowds, but not all crowds are volatile or negative in nature. For example, in the beginning of the socialist movement crowds were asked to put on their Sunday dress and march silently down the street. A more-modern example involves the sit-ins during the Civil Rights movement. Crowds can reflect and challenge the held ideologies of their sociocultural environment. They can also serve integrative social functions, creating temporary communities.

Crowds can be defined as active ("mobs") or passive ("audiences"). Active crowds can be further divided into aggressive, escapist, acquisitive, or expressive mobs. Aggressive mobs are often violent and outwardly focused. Examples are football riots, the Los Angeles riots of 1992, and the 2011 English riots. Escapist mobs are characterized by a large number of people trying to get out of a dangerous situation like the November 2021 Astroworld Festival. Incidents involving crowds are often reported by media as the results of "panic", but some experts have criticized the media's implication that panic is a main cause of crowd disasters, noting that actual panic is relatively rare in fire situations, and that the major factors in dangerous crowd incidents are infrastructure design, crowd density and breakdowns in communication. Acquisitive mobs occur when large numbers of people are fighting for limited resources. An expressive mob is any other large group of people gathering for an active purpose. Civil disobedience, rock concerts, and religious revivals all fall under this category.