In sociology, a peer group is both a social group and a primary group of people who have similar interests (homophily), age, background, or social status. The members of this group are likely to influence the person's beliefs and behaviour.

During adolescence, peer groups tend to face dramatic changes. Adolescents tend to spend more time with their peers and have less adult supervision. Adolescents' communication shifts during this time as well. They prefer to talk about school and their careers with their parents, and they enjoy talking about sex and other interpersonal relationships with their peers. Children look to join peer groups who accept them, even if the group is involved in negative activities. Children are less likely to accept those who are different from them.

Cliques are small groups typically defined by common interests or by friendship. Cliques typically have 2–12 members and tend to be formed by age, gender, race, and social class. Clique members are usually the same in terms of academics and risk behaviors. Cliques can serve as an agent of socialization and social control. Being part of a clique can be advantageous since it may provide a sense of autonomy, a secure social environment, and overall well-being.

Crowds are larger, more vaguely defined groups that may not have a friendship base. Crowds serve as peer groups, and they increase in importance during early adolescence, and decrease by late adolescence. The level of involvement in adult institutions and peer culture describes crowds.