Adolescent externalizing and health risk behaviors are some of the leading causes of morbidity and mortality among young people (Blum & Qureshi, 2011;Kann et al., 2018) and are of significant public health concern. Adolescence is a key period for understanding these types of behaviors, as they tend to emerge and peak in this stage (Claxton & van Dulmen, 2013;Krieger et al., 2018). Importantly, adolescence is not only a key risk corridor for risky and problem behaviors, but also for entry into new social and digital spaces; most social networking sites (and their regulators) set age 13 as the age at which youth can have their own accounts (Jargon, 2019). Co-construction theory (Subrahmanyam et al., 2006) asserts that adolescents create (and co-create) their online worlds and experiences to match developmental needs, and thus we should not be surprised that adolescents' developmentally appropriate affinities for risk taking, boundary testing, and affiliation would all manifest in some form in digital spaces, and that digital activities and offline behaviors would be mutually influential.How youth digital media use and externalizing/risk-taking behaviors intersect is somewhat more complicated. In many domains, adolescent rates of health risk behaviors (substance use, sexual risk taking, violence perpetration) are at their lowest levels in decades (Lewycka et al., 2018;Twenge & Park, 2017), which some have asserted may be related to the proliferation of digital media and displacement of time (previously spent engaging in risk behaviors) in favor of time online and new forms of leisure, entertainment, and relationship formation (Kraut et al., 1998). Others have posited that youth engagement in online communities allows for covert or hidden coordination or reinforcement of deviancy and rule breaking, and thus technology may be linked with increased problem behavior (Ehrenreich & Underwood, 2016). In fact, the associations are not always straightforward, and thus this chapter seeks to summarize and integrate the research findings that have been published to date on these mutual influences and the mechanisms that underlie them.