Life is a series of transitions across institutions, relationships, and roles. People age, develop new identities, assume new roles, and change associates, friends, and even family. Joining a gang is one of the critical transitions that may occur in the life course. Although group processes and structures are both central to this book, up to this point we have paid more empirical attention to the latter. We have compared the members of gangs to inmates who were not affiliated with gangs. In addition, we examined the form and function of the different structures of gangs behind bars, including their role in the governance of prisons as well as misconduct and victimization. In this chapter, we focus on a crucial aspect of group process: joining a gang. If transitions into new identities, roles, and statuses are consequential for the life course, we ought to learn about how such transitions unfold, especially if they occur in places like prisons that are tasked with the job of rehabilitation.