The purpose of this randomized controlled trial is to compare the efficacy and potential biological mechanisms of action of 10 sessions of a web version of Prolonged Exposure (PE), Web PE, delivered over 8 weeks to 10 sessions of Present Centered Treatment (PCT) delivered over 8 weeks by a therapist in 120 active duty military personnel and veterans with PTSD. Up to 170 individuals will be consented to obtain data from 120 for analysis. Participants will be assessed at pre treatment, mid treatment, and 1, 3 and 6 months after treatment completion.