Attention-deficit/hyperactivity disorder (ADHD) represents a powerful entry point for developmental approaches to psychopathology due to its substantial role in early emergence of major life problems. One key issue concerns the role of early environmental risks in etiology and maintenance in the context of genetic liability. Here, psychosocial aspects of development need more attention. A second key issue is that phenotypic heterogeneity requires better resolution if actionable causal mechanisms are to be effectively identified. Here, the interplay of cognition and emotion in the context of a temperament lens is one helpful way forward. A third key issue is the poorly understood yet somewhat striking bifurcation of developmental course in adolescence, when a subgroup seems to have largely benign outcomes, while a larger group continues on a problematic path. A final integrative question concerns the most effective conceptualization of the disorder in relation to broader dysregulation. Key scientific priorities are noted. Expected final online publication date for the Annual Review of Developmental Psychology, Volume 2 is December 15, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.