200 persons with depression are recruited via internet depression forums devoted to depression and are randomly assigned either to the online program deprexis or to a wait-list control condition. All participants receive free-of-charge online access to deprexis either immediately or with an eight week delay.

Prior to intervention and eight weeks later, both groups are assessed via an anonymous online survey which was implemented using the software package OPST®. The survey consists of different questionnaires. The Beck Depression Inventory (BDI) represents the primary outcome. It is assumed that the severity of depressive symptoms will improve to a significantly greater extent in the deprexis than in the wait-list control condition in the course of eight weeks.

Deprexis is a web-based intervention that can be obtained online (www.deprexis.com). Deprexis encompasses 10 content modules with a strong focus on evidence-based cognitive-behavioral techniques, either from its so-called first (behavior-oriented), second (cognitive-oriented) or third wave (e.g. mindfulness and acceptance): (1) psychoeducation, (2) behavioral activation, (3) cognitive modification, (4) mindfulness and acceptance, (5) interpersonal skills, (6) relaxation, physical exercise and lifestyle modification, (7) problem solving and (8) expressive writing and forgiveness, (9) positive psychology and (10) dreamwork and emotion-focus interventions. The ten modules are framed by one introductory and one summary module. Communication is carried out online via simulated dialogues. Patients have to respond to narrative text messages from the virtual therapists in multiple-choice response fashion, enabling them to express doubt, to affirm a particular message or to request more information. Text messages are aided with drawings, photographs and animations. The program is adaptive and selects exercises and contents on the basis of the subject's response. The modules are sequential and each module refers and builds upon previous one. Modules can (and should be) repeated within the intervention period.

Inclusion Criteria:

• registration in a moderated online forum exclusively for affective disorders
• (externally) confirmed diagnosis of a unipolar depression
• informed consent (provided online in line with regulations by the Hamburg Department of Data Security)

Exclusion Criteria:

• unable to provide informed consent
• diagnosis of bipolar or schizophrenia spectrum disorders
• substantial neurological impairment (e.g. dementia)
• acute suicidal tendencies as assessed with the Suicide Behaviors Questionnaire-Revised (SBQ-R). Subjects excluded for this reason are provided various emergency contacts and phone numbers.