The aim of this study is to increase understanding of the effectiveness and efficiency of psychological treatment for adult patients with posttraumatic stress disorder -PTSD- and to make it more personalized. Key questions: 1. Which predictors of treatment success influence treatment outcome of patients with PTSD who receive the three psychotherapeutic treatments investigated in this study? 2. Which specific moderators can be identified with regard to the different psychotherapies (Eye Movement Desensitization and Reprocessing -EMDR-; Prolonged Exposure -PE-; and Interpersonal Psychotherapy -IPT- in the second phase)? 3. In patients with PTSD, does offering another proven effective form of trauma-focused psychotherapy (PE after EMDR, or EMDR after PE) improve symptoms following insufficient response to a first trauma-focused treatment? 4. Is switching from a trauma-focused therapy to a non-trauma-focused treatment (IPT) a more effective strategy for dealing with non-response to a first proven effective psychotherapeutic treatment compared to switching to another trauma-focused therapy? 5. Are there differences in treatment tolerance and differences in dropout rates between PE, EMDR and IPT? Secondary goals: * Investigating the extent to which therapist allegiance to a specific therapy method affects outcomes; * Investigating whether the quality of therapy implementation or the treatment integrity ('adherence/ competence') affects treatment outcomes; * Investigating how much the quality of the therapeutic alliance influences outcomes. Participants receive treatment and will complete questionnaires. The study is conducted in two phases. Its aim is to compare two different trauma-focused treatments (EMDR and PE) for patients with PTSD to one another and with a nontrauma-focused psychotherapy (IPT) and to investigate possible predictors and moderators for treatment success. Patients will first be randomized to PE or EMDR in the first treatment phase. After this first phase, non-responders are re-randomized for a second phase of treatment. They receive either the alternative phase 1 trauma-focused psychotherapy or IPT as non-trauma-focused therapy. In phase 1 researchers will compare the PE and EMDR group to see which treatment is most effective for whom. In phase 2 researchers will compare the trauma-focused treatments (PE and EMDR group) with the nontrauma-focused treatment (IPT group) to see which treatment is most effective for whom.