This chapter presents a developmental perspective on historical increases in youth non-affiliation. First, it describes changes that happen in religiousness across adolescence, focusing on quantitative and qualitative changes. In terms of quantitative changes, generally, religiousness declines across adolescence. In terms of qualitative changes, a sizable minority of youth leave religion during adolescence. Second, the chapter explains these changes, focusing on individual and contextual predictors of quantitative and qualitative changes. In terms of individual predictors, it focuses on gender, ethnicity, personality, behavior, and various dimensions of religiousness. In terms of contextual predictors, it focuses on the family, the religious community, and peers. Third, the chapter identifies ways parents and religious leaders might be able to optimize adolescent religious development. Its suggestions are grounded in self-determination theory, which articulates characteristics of developmental contexts that may help or hinder internalization of religiousness. Finally, the chapter connects individual religious development to historical trends.