Decades of research on adolescence has demonstrated that contexts such as families, peer groups, schools, and neighborhoods play an important role in adolescent development (Petersen, 1993;Steinberg & Morris, 2001). To these well-accepted contextual influences, we should add mediaboth mass media (e.g., television, films, and music) as well as new digital media, which include the Internet (e.g., websites, online forums and communities, and video-and image-sharing platforms), communication applications/platforms (e.g., social media and messaging apps), and electronic games. Survey data suggest that digital media have become ubiquitous in young people's lives (Anderson & Jiang, 2018;Rideout & Robb, 2019); of particular note is that a majority of US adolescents now have access to a computer or smartphone, with 95% reporting access to a smartphone and 45% reporting that they are online almost all the time (Anderson & Jiang, 2018).Research to date suggests that adolescents primarily use digital media for information, communication, and entertainment, with peer interaction and communication becoming especially popular (Anderson & Jiang, 2018;Subrahmanyam & Greenfield, 2008;Valkenburg & Peter, 2011). In prior work, we have suggested that new digital worlds should be considered an important developmental context during adolescence (Subrahmanyam & Šmahel, 2011;Subrahmanyam et al., 2006). Not only are digital media an important social context, but they have also become instrumental in adolescents' interactions with other key contexts such as friends and families. Relationships with friends and families are predictive of health and wellbeing during adolescence (Moore et al., 2018) and it is important to understand the impact of youths' digital media use on their psychological well-being and mental health. 1 This handbook brings together the multidisciplinary scholarship on adolescent social media use and mental health, and critically evaluates the extant research to provide a blueprint for future research. In this introductory chapter, we first provide an overview of the definitions and terminology related to digital media and social media, an overview of adolescents' digital media use at the time of this writing, and a brief historical account of the study of adolescents' social media use. In the second part of the chapter, we describe some of the key methodological and conceptual issues pertaining to adolescent digital media use research.