There have been dramatic changes in internet access, mobile phone use, and social media activity over the past few decades. How has this increased online exposure affected adolescents' mental health? This chapter provides an insightful in-depth discussion of the existing evidence examining associations between mental health and technology use including depression, anxiety, body dissatisfaction, attention-deficit disorders (and risks of distraction), and addictive behaviors as well as the impacts of risky online communities on adolescents' mental health, focusing on networks promoting pro-eating disorder behaviors and pro-suicidality. Helpful recommendations for parents, educators, and providers are included in each section.