This bulletin presents research on why youth join gangs and how a community can build gang prevention and intervention services. The author summarizes recent literature on gang formation and identifies promising and effective programs for gang prevention. The following are some key findings: (1) Youth join gangs for protection, enjoyment, respect, money, or because a friend is in a gang; (2) Youth are at higher risk of joining a gang if they engage in delinquent behaviors, are aggressive or violent, experience multiple caretaker transitions, have many problems at school, associate with other gang-involved youth, or live in communities where they feel unsafe and where many youth are in trouble; and (3) To prevent youth from joining gangs, communities must strengthen families and schools, improve community supervision, train teachers and parents to manage disruptive youth, and teach students interpersonal skills. (Contains 1 table, 2 figures and 7 endnotes.)