Adolescence is a time of increased developmental stress and associated risk for psychopathology. At the same time, today’s adolescent is almost constantly digitally connected, and the online space has been largely overlooked by researchers as a context for youthful coping. As a result, the current thesis includes three empirical studies to address critical gaps in our measurement of adolescent coping (online or off), how adolescents look to the digital arena as they navigate day-to-day life, and the short-term influence of online coping on adolescents’ emotional well-being. First, Study 1 sought to characterize how scholars are using technology to measure adolescent coping via ambulatory assessment (AA), and to delineate associated advantages and challenges of varying approaches. Previous published research has remained challenged by how best to conceptualize, measure, and analyse adolescents’ coping in situ. Thus, drawing from 60 adolescent AA coping studies, Study 1 called for scholars to revisit coping theory in their study designs to ensure they tap their focal aspect of the adolescent coping process. Study 1 also provided key lessons and recommendations for scholars seeking to deploy AA methods in their pursuit of measuring coping. Second, Study 2 sought to establish a foundation for adolescent online coping. This study brought together data from adolescent focus groups (Study 1; n = 16), experience sampling (Study 2; n = 156), and young adult surveys (Supplementary data; n = 213). Study findings validated adolescents’ online coping as a strategy that youth widely endorse in the face of daily stress. Specifically, in line with common coping facets, and drawing on the communications literature, three online coping strategies emerged: online emotional support seeking, information seeking, and self-distraction. Moreover, findings suggest negative linear effects for these online coping strategies; when conceived as an individual difference construct, whereby more online coping was associated with worse emotional reactions to stress in daily life. Third, Study 3 sought to explore the impact of adolescents’ online coping using a more fine-grained approach. Specifically, by tying ambulatory assessments of online coping to momentary stress reports, this study allowed for the analyses of the full coping process—stress, coping, response—within an in-situ framework. Moreover, this study capitalized on momentary coping reports in a subset of youth (n = 115; 1,241 timepoints) to assess both linear and non-linear associations with short-term emotional well-being. Findings indicated a negative linear impact of momentary online coping, such that more emotional support seeking, information seeking, and distraction online were associated with worse emotional responses. However, testing of non-linear associations indicated better fitting models across the board, and a robust pattern of results. Here, moderate levels of online coping had a clear positive impact on adolescents’ emotional recovery from stress. All told, thesis findings point to the important arena of technology to support adolescents' coping and associated well-being. Thesis studies contribute to the literature in several arenas, including a much-needed scoping review of the existing AA coping literature, and a robust validation of the online coping construct. Further and most importantly, studies make clear that online coping has an effect on adolescents’ emotional well-being, and when used in moderation, may be beneficial to their emotional functioning.