This document summarizes key points about peer groups from childhood through adolescence. It discusses how peer groups evolve from simple playmates in early childhood to more complex social organizations in school-age years and adolescence. Peer groups provide social support and influence development by encouraging social skills, values, and behaviors. Both positive and negative outcomes can result from peer group interactions, such as collaboration or bullying/gangs. The document also examines characteristics of bullies and victims as well as reasons why gangs may form.