After decades of rigorous outcome research, we can confide in the scientific evidence that psychotherapy for adolescent depression works. However, knowing that psychotherapy works, in general, does not yet indicate how and why. Today, these more complex questions challenge the field, as better adolescent-specific treatments are needed to answer pressing societal and clinical needs. The specific nature of adolescent depression and the underlying mechanisms operating during the therapeutic process, particularly require further study. Person-specific analyses on intensive longitudinal data in that sense promise a unique contribution, enabling to capture not only between-person but also within-person variability. Recent developments in research methodology and electronic devices (e.g., smartphone applications), moreover, make this approach particularly suited for research with adolescents. To date, no systematic application to the study of psychotherapy for adolescent depression was done, however. Therefore, this study will apply innovative idiographic research methodology (both experience sampling and mixed methods) that allow fine-grained exploration of both depression and psychotherapeutic changes. Aiming to contribute to better scientific knowledge on how and why psychotherapy works for depression in adolescence, we will 1) identify idiographic patterns in adolescent depression experiences and 2) identify ideographic change patterns during psychotherapy.