This chapter explains a study which researchers in collaboration with program managers and program providers developed an assessment to support secondary gang prevention for the Los Angeles Gang Reduction and Youth Development (GRYD) Program. It describes steps taken by the researchers to identify an empirically defensible set of risk factors, develop a strategy to determine reasonable cutpoints to define high risk on each factor, and provide program providers with the feedback needed to selectively enroll high-risk youth in the GRYD Secondary Prevention Program. The chapter examines spheres of influence rather than domains because of conceptual and operational differences in the available data. The familiar recruitment and referral networks shifted to more efficient identification of high-risk youth but the data reviewed here confirm that the assessment was still needed to select high-risk youth for secondary gang prevention. These approaches to working effectively with the high-risk youth are being evaluated.