Major depressive disorder (MDD) is ranked one of the most burdensome disorders for adolescents worldwide. There is an urgent need for accessible interventions, as many adolescents suffering from MDD do not receive treatment. Internet-delivered interventions remove barriers to seeking and receiving treatment, and internet-delivered cognitive behavioural therapy (ICBT) has been found to be effective for adolescent MDD. However, not all are helped by ICBT and treatment alternatives are needed. Internet-delivered psychodynamic therapy (IPDT) has previously been tested with promising effects in adults, but no studies have assessed its efficacy for adolescents. Furthermore, little is known about mechanisms of change in internet interventions targeting adolescent MDD and how adolescents experience the therapeutic relationship in IPDT. This thesis aims to evaluate efficacy as well as processes and experiences of IPDT, from different perspectives.

Study I investigated effects of IPDT for adolescents aged 15–18 (n = 76) suffering from MDD, compared to control condition. IPDT was found to be significantly more effective than a supportive control condition on reducing depression (d = 0.82). Furthermore, moderate to large significant effects in favour of IPDT were found for comorbid anxiety, emotion regulation and self-compassion.

Study II explored participants’ (n = 18) experiences of the psychotherapeutic relationship in IPDT. Semi-structured interviews were analysed using thematic analysis. Four themes were created: “a meaningful and significant relationship with someone who cared”, “a helping relationship with someone who guided and motivated me through therapy”; “a relationship made safer and more open by the fact that we didn’t have to meet”; and “a nonsignificant relationship with someone I didn’t really know and who didn’t know me”.

Study III examined the relationship between therapeutic alliance, emotion regulation and outcome week-by-week in IPDT and ICBT for adolescent depression (n = 272). Results showed that therapeutic alliance, as rated by both therapist and participant, predicted outcome in depressive symptoms week-by-week in both treatments. Furthermore, this relationship was mediated by emotion regulation, again in both treatments.

In conclusion, results from this thesis indicate that IPDT may be a viable treatment option for adolescent depression. Furthermore, it is possible to form a close and safe relationship between therapist and participant, experienced as important for the psychotherapeutic process by many participants. Lastly, therapeutic alliance plays an important role in both IPDT and ICBT for adolescent depression, partly through its effect on emotion regulation.