After decades of rigorous outcome research, we can confide in the scientific evidence that psychotherapy for adolescent depression works. Today, the more complex question of how and why psychotherapy works however challenges the field, as better person-tailored treatments are requested. The specific nature of adolescent depression and the underlying mechanisms operating during the therapeutic process, particularly require further study. An increasing use of person-specific research approaches can thereby be observed, i.e., methods that enable to capture both between-person and within-person variability. Innovative developments on the level of research methodology and electronic devices, moreover, make this approach particularly suited for studying therapy in an adolescent population. To date, this however has not been done. Therefore, the presented I-MAP Study will employ innovative idiographic research methods (both longitudinal 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, three key objectives are proposed: The I-MAP Study will 1) identify idiographic networks in adolescent depression experiences prior to therapy (ID-MAP), 2) identify ideographic change patterns over the course of therapy (IC-MAP), 3) identify which idiographic change patterns are related to good and poor therapy outcome (IO-MAP).