This project will be done in three stages. The first stage will look at MVP-obtained data and electronic health record (EHR) data to implement methods for identifying combat-exposed case patients with PTSD and combat-exposed control patients without PTSD. The second stage will assemble and validate a study population of 20,000 participants "including 10,000 combat-exposed Veterans with PTSD as cases and 10,000 combat-exposed Veterans without PTSD as controls. The third stage will conduct genetic analyses ("genotyping") comparing the cases to controls, to identify genes associated with increased risk of developing the condition.