This study is a prospective, randomized (1:1), controlled multisite study of MBSR in 180 veterans with PTSD. The investigators will compare the clinical outcomes of MBSR to a credible group therapy control condition called Present Centered Group Therapy (PCGT). The investigators will to evaluate the efficacy of MBSR in the treatment of PTSD, as defined by the Clinician Administered PTSD Scale (CAPS), a gold standard measure that is clinically relevant to PTSD. The CAPS will be rated by a trained assessor who is blind to the whether or not the participant receives the MBSR or PCGT intervention. Secondarily, the investigators will evaluate the effects of MBSR on mindfulness, depression, PTSD symptom clusters, rates of response and evaluate the safety and tolerability of MBRS. The investigators will also explore the subjects' acceptance of and satisfaction with MBSR, whether or not the subjects' treatment preference influences outcome; the stress-immunological-biomarkers as they relate to treatment outcome; and the durability of therapeutic effects of MBSR in the treatment of PTSD.