This article examines the who, what, where, when, why, and how of gang joining. The question of what youth join when they join gangs speaks to the contested nature of gang definitions and types and the consequences of gang membership, specifically heightened levels of offending and victimization. The type of gang and the obligations of membership influence the joining process. Where youth join gangs, namely, the neighborhood and social context, also impacts individual opportunities and preferences for joining. When youth join gangs is considered in a developmental sense, to include both adolescent and adult onset, in order to account for continuity and change in individual levels of immersion or “embeddedness” in gangs across the life course. Who joins gangs provides a profile of gang membership grounded in the well-documented risk factors for gang membership, but limited by problems of prediction. Why youth join gangs speaks to the push and pull factors for membership, the appeal of gangs, and the selective incentives they offer. Still, motivations for gang membership cannot fully explain selection into gangs, nor can general theories of crime that do not necessarily fit with general knowledge of gangs. How youth join gangs, for example, is more complicated than initiation rites. The mechanisms underlying the selection process can be understood through the lens of signaling theory, with implications for practice.