The investigators are seeking people who have been exposed to a traumatic event in the past and have symptoms of posttraumatic stress disorder (PTSD) currently. A person with PTSD may feel significant distress when reminded of a traumatic event or feel depressed, anxious or jumpy.

As a part of this study, participants will receive brain MRIs and office assessments before and after psychotherapy. The investigators provide the gold-standard psychotherapy for PTSD, "Prolonged Exposure", free of charge; additionally participants are compensated for their time during assessment procedures. This study is exploring the brain circuitry involved in improvement in response to psychotherapy.

Inclusion Criteria:

1. age between 18 and 60 years;
2. fMRI scanning eligibility, including no evidence of any form of metal embedded in the body (e.g., metal wires, nuts, bolts, screws, plates, sutures), as these produce artifacts when brain imaging;
3. not currently involved in an exposure-based psychotherapy, in order to be able to measure and interpret the effects of PE on PTSD;
4. must comprehend English well and show non-impaired intellectual abilities to ensure adequate comprehension of the fMRI task instructions and PE treatment;
5. no history of neurological or cardiovascular disorders, brain surgery, electroconvulsive or radiation treatment, brain hemorrhage or tumor, stroke, seizures or epilepsy, diabetes, hypo- or hyperthyroidism, head trauma with loss of consciousness greater than thirty minutes;
6. no regular use of benzodiazepine, opiate, thyroid, anticonvulsant or antipsychotic medications. Patients on stable doses of antidepressant medications will be allowed. Patients for whom antidepressant dosing is being actively titrated will be required to be on a stable dose for 1 month prior to inclusion in the study.

Exclusion Criteria:

• Any contraindication to being scanned in the 3T or 1.5T scanners at the Lucas Center or CNI such as having a pacemaker or implanted device that has not been cleared for scanning at the Lucas Center or CNI.
• Participants will be excluded from the study if there is any lifetime evidence of psychosis, mania, hypomania, or bipolar disorders. Other axis I comorbidities will not be a cause for exclusion.

In addition, subjects will be excluded if they have a significant CNS neurological condition such as stroke, seizure, tumor, hemorrhage, multiple sclerosis, etc.

Patients who have current substance dependence will be excluded from the study. A recent diagnosis of substance abuse is allowable, however, as long as subjects have been abstinent for greater than three months.

• Subjects will be excluded if they are currently in an exposure-based psychotherapy for PTSD.