The rapid growth in popularity of social media platforms has fueled concerns about the possible negative effects of social media use on young people’s mental health and body image. To unravel social media effects, scholars argued that social media activities should be divided into active and passive social media use. It has been hypothesized that active social media use (i.e., creating content) leads to positive effects on mental health and body image, whereas passive social media (i.e., viewing content) leads to negative effects. In some cases, however, active social media use appeared to also lead to more negative feelings about oneself and one’s life, while passive social media use can lead to more positive feelings about these matters. Hence, the active-passive dichotomy seems a rough division that needs further specification. What types of content do young people post online, and what do they passively consume? And how does that affect their mental health and body image? To advance our knowledge and insights on these matters, the overarching goal of this dissertation was to investigate if, when, and how social media use affects young people’s mental health and body image. In Chapter 2, findings of a systematic literature review indicated that how social media use coincides with mental health and body image largely depends on the content of the self-presentation created or passively consumed. For example, sharing negative life events was generally linked to decreased subjective well-being. In contrast, sharing authentic self-presentations was linked with increased subjective well-being. The second study (Chapter 3) examined how ethno-racial identities may predispose differences in publicly posted selfies. Facial recognition was used to classify the selfie-takers’ ethno-racial identity on over 3000 publicly posted selfies. Findings revealed that individuals varying in ethno-racial identity generally differed in their online self-presentations. For example, Asian and Hispanic selfie-takers posted more of their context and less of themselves than black and white selfie-takers. The third study in this dissertation examined how the prevalence of posting and seeing different photo-types was linked with mental health and body image (Chapter 4). This study included more than 400 adolescents. Findings of this study indicated that, for example, posting more authentic photos of oneself is associated with higher levels of mental health, whereas posting edited photos is associated with more body shame. The last study examined the potential longer-term impact of social media use (Chapter 5). This study included more than 400 Japanese adolescents and more than 500 Dutch adolescents. The adolescents completed three online surveys, with one-month intervals in between. Findings of this study indicated that both a higher frequency of posting and being exposed to authentic photos can induce positive outcomes for mental health and body image, whereas both a higher frequency of posting and being exposed to edited photos of others was found to relate to negative outcomes. Results were, however, different between Dutch and Japanese adolescents. The results of the studies in this dissertation generally conclude that time-based approaches of social media use reflect an oversimplification to interpret the possible effects on mental health and body image. The findings of this dissertation all advocate that there is a need to apply an integrated content-specific and behavior-specific approach. Moreover, social media effects are conditional, meaning that social media effects cannot be attributed to all adolescents and may vary across cultural context. Hence, if and when social media impacts mental health and body image largely depends on how and why they use social media as well as the way they respond to their social media use.