Currently, veterans with posttraumatic stress disorder (PTSD) are typically treated with antidepressants which have limited efficacy and yield extremely low remission rates. New and improved treatments are sorely needed, especially in light of the inadequate evidence to support the efficacy of most pharmacologic and most psychotherapy treatments for PTSD. Complementary and Alternative Medicine practices, such as meditation, may fill this void. Several other illnesses often found in association with PTSD, such as chronic pain, anxiety, and depression, have shown positive response to meditation, specifically Mindfulness Based Stress Reduction (MBSR). MBSR is manualized program involving 8 weekly classes and a single 6-hour silent retreat session based on a systematic procedure to develop enhanced non-reactive awareness of the moment-to-moment experience of perceptible mental processes. MBSR is well tolerated, already well disseminated. MBSR has been shown to be effective in reducing conditions that are commonly associated with PTSD, including pain, depression, anxiety and panic, and insomnia. The investigators are conducting a multisite study of Mindfulness Based Stress Reduction (MBSR) compared to Present Centered Group Therapy (PCGT) for the treatment of PTSD. The primary aim of this study will be to determine how well the MBSR form of meditation, i.e. MBSR, works to treat symptoms of PTSD in veterans compared to PCGT. The investigators will evaluate the effects of treatment on levels of mindfulness, depression, PTSD symptom clusters, and response rates. The investigators will evaluate veterans' satisfaction and acceptability of the MBSR intervention. Additionally, biomarkers will be utilized to aid in our understanding of the pathophysiology of meditation and explore the relationship between treatment outcome and neuroimmune response.