The current study will modify an already existing therapy for individuals with substance use disorders to address PTSD in women enrolled in SUD treatment who suffer with both PTSD and SUD. Mindfulness meditation has been shown to help individuals to cope with stress and regulate emotions. Through meditation practice women will experience less distress related to PTSD symptoms and reduced substance use in response to emotional triggers. Women enrolled in intensive SUD treatment at a community program will be randomized to receive either 8 weeks of 90 minute Mindfulness Based Relapse Prevention (MBRP) group sessions plus treatment as usual (TAU) or TAU alone. The MBRP group sessions will replace 90 minutes of TAU group therapy. Measures of feasibility of implementation, acceptance and adherence will be obtained. Preliminary efficacy for substance use and PTSD symptom severity will be measured at post treatment and at 3- and 6- months' follow-up.