In the project ‘Cognitive-behavioural treatment for female patients with PTSD and SUD’ (PI Ingo Schäfer, Hamburg) N = 342 female patients with PTSD and SUD are included in a multi-centric study in Hamburg, Cologne, Essen, Bielefeld, and Hannover to compare the effects of SS (Najavits, 2002) to another standardized CBT intervention (relapse prevention training; RP), and treatment as usual (TAU). SS is suitable for patients of both genders, but this study will focus on female patients. While single-sex studies are clearly limited with regard to the generalizability of their findings (Kristman-Valente & Wells, 2013), this decision was made because of the higher prevalence of the comorbidity of SUD and PTSD in females (e.g. Driessen et al., 2008; Schäfer & Najavits, 2007), and the suggestion of some authors that trauma treatment should preferably be offered in gender-specific groups (e.g. Greenfield et al., 2007). Patients in the SS group receive 14 weekly sessions (90 min each) in groups of four to eight patients. The intervention is offered in addition to TAU. The same number and duration of sessions is offered to patients in the ‘RP + TAU’ group. Patients in the TAU group are offered individual SUD treatment on a weekly basis and may obtain any treatment they normally would (detoxification, rehabilitation, individual psychotherapy, self-help groups, etc.). While the primary outcomes are PTSD symptoms as measured by the PTSD Symptom Scale Interview (PSSI), the secondary outcomes include substance use and other domains of psychopathology. The primary hypothesis consists of three two-group comparisons: (1) Enhanced treatment (SS + TAU) is more effective with regard to reduction of PTSD symptoms at six-month follow-up than TAU alone; (2) the same is expected for relapse prevention training (RP) + TAU as compared to TAU alone; and (3) SS + TAU will be at least as effective as RP + TAU.