This document discusses the ecology of peer groups. It defines a peer group as composed of individuals of roughly equal age and background. It describes four main types of peer groups: elites, athletes, academics, and deviants. Peer groups influence members through socialization, shaping self-esteem and gender roles. While peer groups provide belonging and support, they can also negatively pressure behavior if members have bad influences. Peer groups have a structure that includes peers, crowds, cliques, and friends.