The number of patients seeking treatment for PTSD in the mental health services of the capital region of Denmark (MHS-CRD) has been increasing significantly. This creates a need for tools to increase treatment outcomes at low-to-no cost and underlines the need for research into the effectiveness of existing treatments in naturalistic clinical settings. Therefore, the aims of this thesis were to: 1) investigate the effectiveness of PTSD treatment provided in the MHS-CRD in a naturalistic sample and 2) to investigate the feasibility of implementing a mental health mobile application (mHealth) for PTSD-patients in the MHS-CRD. The two studies included in the thesis showed a limited effectiveness of the existing treatment in the MHS-CRD, as well as a significant gender difference in treatment outcome. Further, the studies illustrated challenges in implementing mHealth interventions for PTSD-patients, and a need for further research into how to optimize the effect of these in psychiatric PTSD-populations.