In the present article, we review the theory and research on religious deconversion with a focus on adolescence and young adulthood. First, we present the relevant terminology (e.g. religious deconversion, religious disaffiliation, and religious deidentification) and statistical trends (e.g. the prevalence of religious Nones and Dones). We define religious deconversion as any movement away from religion. Religiosity decreases across adolescence and into young adulthood, and these developmental periods also have heightened rates of religious deidentification, at least in many Western cultures. Second, we review the theory and research on processes, predictors, and outcomes of religious deconversion. Deconversion is discussed both in terms of individuals becoming less religious and possibly deidentifying from religion. In terms of processes of deconversion, we discuss various theoretical models and self-expressed reasons for deconversion. Religious deconversion tends to progress in phases, often initiated by religious and spiritual struggles. In terms of predictors, we identify predictors at the level of the individual (e.g. demographics, personality, norm-violating behavior, and dimensions of religiosity), family (e.g. parent and family religiosity, family dynamics such as parenting styles, and religious socialization), peers (e.g. peer religiosity and religious conversations with peers), religious communities (e.g. supportiveness, engagement, and dialogue), formal education, and social media. In line with developmental systems perspectives, religious deconversion is a complex and dynamic process involving numerous individual and contextual factors. In terms of outcomes of deconversion, we identify potential individual and relational outcomes that are considered both negative and positive. Finally, we outlined 10 promising future directions for research on religious deconversion.