This is a systematic review of 30 years (1988-2017) of empirical research on processes of religious/spiritual influence in adolescence. We followed a multi-step process that resulted in 241 studies organized according to eight research questions and the corresponding methods and analyses typically used to address them. We coded these studies based on the dimensions of religiosity/spirituality and the youth outcomes involved. In some cases (quantitative studies of mediation and moderation, as well as qualitative studies) we also coded a third process variable. Results of the coding process revealed a number of interesting patterns. First, religiosity/spirituality is generally adaptive for adolescents, protecting them from negative outcomes (e.g., risk behaviors and mental illness), and promoting positive youth development and flourishing. Nevertheless, in some contexts, religiosity/spirituality may be at least partially maladaptive. Second, there is some evidence, from experimental and longitudinal studies, that relations between religiosity/spirituality and adaptive outcomes are causal. Third, there are numerous complex and dynamic processes by which religiosity/spirituality relate to youth outcomes. In terms of mediation studies, the most salient mediating processes seem to involve religiosity/spirituality dimensions, peers, values/attitudes, and social control/norms. Fourth, religiosity/spirituality is multidimensional, involving various interwoven facets at the individual and ecological levels. Private or personal aspects of religiosity/spirituality (e.g., religious/spiritual importance) tend to be more salient predictors of outcomes than public aspects of religiosity/spirituality (e.g., religious worship service attendance). The results of this systematic review point to promising directions for future research. First, more research is needed studying a broader range of dimensions of religiosity/spirituality, processes of influence, and outcomes. In terms of religiosity/spirituality, much of the prior work has focused on overall religiosity/spirituality, and religious/spiritual behaviors (e.g., worship service attendance). In terms of outcomes, the emphasis has been on religiosity/spirituality protecting against maladaptive outcomes (e.g., substance use). Second, more research is needed examining the role of culture, and using more rigorous methods (e.g., experience sampling, experimental design, longitudinal design, or mixed methods). This systematic review provides a detailed analysis of what is known regarding processes of religious/spiritual influence in the lives of adolescents, and hopefully better positions researchers to move the field forward.