Adolescents spend increasing time using their smartphones but the current understanding of how they use them is limited and relies on self-reported surveys - therefore we need methods that would capture more precisely how adolescents use their smartphones and how it impacts them. One promising way is to combine ecological momentary assessment, which involves intensive repeated measurement with automated data collection from adolescents’ own phones using a custom-built research app. EMA involves intensive repeated data sampling (i.e., several times a day), allowing for the examination of both within- and between-person variability with both reduced retrospective recall and self-report bias. Our contribution to the book discusses in detail how we developed a research app that our participants installed in their own smartphones, and how we tackled the ethical and technological obstacles. Our experience suggests that the ecological momentary assessment method can be successfully applied by combining self-reported data with objective data from mobile phones. This means that we can better understand adolescents’ smartphone behavior and the complex dynamics of effects at both the within- and between-person levels. That is. we can explore differences between individuals, but also differences in one persons’ usage over time. For researchers interested in utilizing similar research design, we add recommendations and lessons learned.