The effects of a monitoring intervention as an add-on to face-to-face Cognitive Behavioral Therapy (CBT) for depression are assessed in this randomised trial. Monitoring consists of Ecological Momentary Assessment (EMA) of symptoms and inter-session processes via mobile phones. Twenty sessions of face-to-face CBT + EMA are compared to twenty sessions of face-to-face CBT (treatment as usual, TAU). 84 patients with mild to moderate depression will be randomised to CBT + EMA or to TAU.

As mobile technologies advance, healthcare is increasingly utilising mobile interventions to accompany traditional interventions such as psychotherapy. Mobile assessments of symptoms and other processes are also used as low-level interventions and are believed to lead to symptom improvements in patients with symptoms of depression, for example. However, robust evidence on the health benefits of monitoring interventions in patients with depression is lacking.

The investigators are interested in the potential benefits of a monitoring intervention as an add-on to face-to-face Cognitive Behavioral Therapy (CBT) for depression. Therefore, the investigators will conduct a randomised trial to investigate the effects of a monitoring intervention consisting of Ecological Momentary Assessment (EMA) of symptoms and inter-session processes via mobile phones. Twenty sessions of face-to-face CBT + EMA will be compared to twenty sessions of face-to-face CBT (treatment as usual, TAU). Participants in the CBT + EMA group will be asked to answer monitoring questions twice daily for two four-week blocks during the CBT treatment. A total of 84 participants with mild to moderate depression will be randomized. Of these, 48 participants will be randomized to CBT + EMA and 36 participants will be randomized to TAU. The primary outcome of interest is change in the Patient Health Questionnaire (PHQ-9) score at four-month follow-up assessment.

Throughout the study, there will be five assessment timepoints: Baseline (before the first therapy session), four weeks after the fourth therapy session, four weeks after the twelfth therapy session, after the last session (post-treatment) and the 4-month follow-up (four months after the last therapy session).

Inclusion Criteria:

• ICD-10 diagnosis of mild or moderate depression, first or recurrent episode (F32.0, F32.1, F33.0, F33.1)
• PHQ-9 ≥ 5
• Possession of smartphone with internet access
• Signed informed consent

Exclusion Criteria:

• Acute or past suicidality; self-reported
• Acute addiction (F10 - F19)
• Dysthymia (F34.1), postpartum depression, or severe depression (F32.3; F33.3)
• PHQ-9 > 20
• Depression not main mental health concern
• Planned onset or planned change in antidepressant medication
• History of psychotic symptoms, bipolar disorder, personality disorders, or organic brain disorders
• Cognitive impairment or physical disability that would hinder psychotherapy or the use of a smartphone

As is common in psychotherapy research, participants and care providers cannot be blinded to the intervention. Only subjective outcomes (i.e., participant-rated) are assessed.