Depression is the most burdensome disorder worldwide in terms of health loss. The potential of internet and technologies to scale up psychological treatment resources is substantial. A blended treatment approach, reducing therapist time and combining sessions with online self-help components, could enhance availability of psychological treatment, while maintaining and possibly enhancing effect of treatment. The aim of this thesis was to develop and investigate the blended treatment approach, in terms of acceptance among key stakeholders, and clinical effect in treatment of depression in the adult and adolescent population.Study I investigated acceptance of the blended treatment approach among mental health care stakeholders by means of a European survey. The results demonstrated that the majority readily accepted blended treatment for management of mild and moderate depression.Study II evaluated blended treatment compared to standard face-toface psychotherapy in treatment of adult depression in a controlled non-inferiority trial. The results showed a similar decrease in depression from both interventions at post-treatment, with decreased levels maintained over six months. Non-inferiority for the blended treatment could not be statistically established.Study III was a controlled non-inferiority trial evaluating blended treatment compared to treatment as usual. The results indicated superiority for the blended treatment at post-treatment and partly at six months. After twelve months the outcomes in the two conditions were similar.Study IV evaluated blended treatment for adolescent depression in a controlled superiority trial, where the therapist time was not reduced, but sessions delivered via chat for improved reach and efficiency. Compared to attention control, the blended treatment significantly reduced depression symptoms, with effects indicated to be maintained over six months.Study V was a controlled superiority trial, evaluating an improved version of the blended treatment used in Study IV to similar methods and in a similar population. In comparison to minimal attention control, the blended treatment significantly reduced depression symptoms at post-treatment, corresponding to a large treatment effect.Across Study II to V, estimates indicated that the amount of therapist time that could be saved in blended treatment, compared to standard psychological treatment, was around 40%.In conclusion, a gradual, blended integration of technology into psychological treatment i) performed well in treatment of adult and adolescent populations, ii) could substantially reduce therapist time in comparison to standard face-to-face psychological treatment, and iii) was accepted by patients as well as other mental health care stakeholders.The thesis demonstrates the potentials of technology-assisted blended treatment models to deliver treatment of depression in the young and adult population in accordance with the current, urgent need to increase availability of psychological treatment as well as increase acceptance ...