Posttraumatic stress disorder (PTSD) is a chronic, debilitating condition with a broad negative impact on physical and mental health functioning. Excellent, empirically-supported interventions for PTSD are readily available to Veterans, but the majority of patients who receive the best available treatments continue to have a diagnosable disorder after treatment. This suggests that complementary interventions may be useful to fully restore functioning. In addition, many patients seek alternatives to psychotherapy or pharmacotherapy. This is evident in the increasing numbers of individuals with PTSD are turning to alternative approaches. At present, however, we lack a good evidence base from which to make recommendations about the use of complementary and alternative approaches. Yoga is very popular practice, touted to enhance physical, mental and spiritual well-being. Many veterans with PTSD have turned to yoga as a part of their recovery plan, often seeking care outside the mental health system. Initial evidence suggests that the practice leads to clinically significant change in symptoms, but results are yet inconclusive, particularly in Veterans. Thus, this project will provide additional data as to efficacy of yoga as compared to physical activity for Veterans with PTSD. One reason that the yoga literature is yet inconclusive may be the tremendous heterogeneity in the way in which the yoga is implemented. By understanding the mechanisms by which yoga affects PTSD symptoms, universal change processes can be identified. This project, therefore, will examine two different theoretical models that may account for yoga’s clinical effect. With 30 million Americans practicing yoga, dissemination of a specific protocol would not be practicable. Nonetheless, guidelines for trauma-sensitive yoga, which focus primarily on acceptability of the practice for traumatized individuals, appear to be associated with better outcomes for those with PTSD. This work would facilitate the extension of such guidelines to include practice features that optimize active mechanisms of change in order to that maximize the impact of yoga on PTSD symptoms. The proposed study is a randomized controlled trial involving 192 Veterans with PTSD from the San Diego area. These Veterans will be randomized to receive a manualized hatha yoga intervention or a supportive physical activity control condition. Both interventions will be delivered by trained instructors in a group setting over a 12-week period. Assessment of clinical outcomes and methodological variables will take place before, during, immediately after, and 3-months after the intervention. Analyses will focus on change in the intervention groups over time and the degree to which change is associated with the candidate mechanisms. Results of this study have the potential to inform the way in which yoga is delivered to Veterans as well as to increase knowledge about the underlying processes by which PTSD can be ameliorate...