Adolescent major depressive disorder (MDD) is a common and debilitating disorder, associated with clinically significant distress and impairment in functioning. A vast array of negative consequences of adolescent MDD have also been found to extend into adulthood. Still, the majority of affected youths do not receive adequate treatment. Internet-delivered interventions address many barriers to treatment, for instance stigma and lack of qualified psychiatric care. However, the most studied internet-delivered treatment, internet-delivered cognitive behavioural therapy (ICBT), leads to clinically meaningful change in somewhat less than 50% of participants. These results indicate the need for treatment alternatives. This thesis consists of three empirical studies examining a newly developed such alternative, internet-delivered psychodynamic treatment (IPDT). Study 1 (n = 272) was a randomised controlled trial, testing whether IPDT was non-inferior to ICBT for depressed adolescents (15–19 years). Results indicated that both treatments were effective, with large within-group effects, and that IPDT was non-inferior to ICBT. No significant differences were noted on primary or secondary outcomes in the intent-to-treat analyses. Study 2 (n = 67) tested emotion regulation as a baseline predictor of rate of change, and whether intra-individual change in emotion regulation was a mechanism of change in IPDT. Results indicated that patients with relatively more severe deficits in emotion regulation had a steeper trajectory towards improvement. Intra-individual changes in emotion regulation also predicted improvements in the subsequent week, indicating that emotion regulation acted as a mechanism of change in IPDT. Study 3 (n = 66) tested whether sudden gains (SGs) and large intersession improvements (LIIs; defined as SGs without demanding symptom stability pre- and post-gain) were associated with superior outcome. LIIs were associated with improved outcome at post-treatment and at follow-up, whilst SGs were not. Overall, the above findings suggest that IPDT is a viable alternative to ICBT for depressed adolescents, that IPDT partly works through increases in emotion regulation and that patients who improve suddenly, between consecutive weeks in IPDT, are more likely to benefit from treatment.