Posttraumatic stress disorder (PTSD) has been identified in 10-20% of U.S. military service members (SMs) returning from Afghanistan and Iraq, and is associated with impaired physical health, mental health, and overall functional status. Resilience has often been spoken of, yet proven methods for promoting resilience, and protecting SMs from PTSD, have yet to be established. One easy and effective way of identifying those at higher risk for development of PTSD is through the one-page, 17-item self-administered PTSD Checklist (PCL). The total score can range from 17 to 85, and a score of 50 is the best cut-off for making a diagnosis of PTSD, but there are many military SMs who have impaired function associated with scores below this level. There have also been some high-profile fatal shootings on military installations in recent years, including Fort Hood, TX, and the Washington Naval Yard in the District of Columbia, and such incidents tend to be followed by persistent symptoms in many individuals who were in the vicinity at the time. There is limited recent evidence to suggest that lower intensity intervention might be effective in those with subthreshold PTSD, even though it does not necessarily successfully treat fully developed PTSD. We will therefore conduct a randomized controlled clinical trial involving 144 individuals who are eligible for care in the military healthcare system who report subthreshold PCL scores (28 to 49), either within five years after their return from Afghanistan or Iraq, or after being affected in some significant way by a terrifying incident such as a bombing or shooting, or a major natural disaster that results in significant loss of life such as a hurricane, tornado, or earthquake. Seventy-two participants will be assigned to the resilience enhancement group, which will be initiated with a single 90-minute introduction to both cognitive behavioral therapy (CBT) techniques and the use of smart-phone based applications (apps) that promote increased resilience through relaxation, biofeedback, psychoeducation and other techniques. Over the subsequent 6 weeks, daily text messages to all participants will accentuate the positive, including providing recommendations on beneficial activities, fostering behavioral changes, and in particular encouraging the completion of smart phone-based applications (apps). The control group (n=72) will be provided with an initial informational session providing them with details of where they can get help if they have worsened symptoms over time. They will receive daily texts with aphorisms for 6 weeks. Symptoms will be re-assessed with the PCL by personal computer (PC), tablet or smartphone based access to the NIH Clinical Trials Survey System (CTSS) for both the resilience enhancement and control groups at the end of each week in the 6-week training period. In addition the GAD, PHQ-9, and PCL will be assessed at baseline and 6weeks and at 3, 6 and 12 months. We anticipate that this relatively inexpensive resilience enhancement program will reduce the incidence of PTSD over the initial 6 weeks by 20% compared to the control group, and that this improvement will be maintained over the subsequent year. We further expect that the training program will be associated with a reduction in the incidence of PTSD by at least 10% over the follow-up period. Our power estimates indicate that with 72 participants each in the resilience enhancement and control groups, we will have 90% power to identify a significant difference between the two groups with 95% confidence (p \< 0.05). For approximately 20% of participants (n=30; 15 in each group) we will perform psychophysiologic measures and functional MRI of the brain using the Affective Stroop paradigm at baseline and again 3-6 months later to assess for changes with resilience enhancement.