Participants with and without PTSD will be enrolled in this 12-week, open label study. Following a screening visit to determine eligibility, participants with PTSD will be offered a choice of either CBT or SSRI treatment. Those participants who choose CBT will attend 16 therapy sessions. The first phase of therapy will focus on the development of emotional and interpersonal regulation skills. The second phase will use a modified form of prolonged exposure therapy, which has been effective in reducing symptoms of PTSD. Participants who elect to receive medication will take sertraline, an SSRI that has been safe and effective in treating PTSD. These participants will attend 12 treatment sessions. Pre- and post-treatment fear response will be assessed in all participants using fMRI scans to measure brain responses and using saliva samples to test cortisol levels. All participants will also self-administer saliva samples at various points during the 3 days prior to fMRI scanning. Other outcomes will be assessed throughout the study using questionnaires.