The authors reviewed 29 studies that provide prevalence estimates of posttraumatic stress disorder (PTSD) among service members previously deployed to Operations Enduring and Iraqi Freedom and their non-U.S. military counterparts. Studies vary widely, particularly in their representativeness and the way PTSD is defined. Among previously deployed personnel not seeking treatment, most prevalence estimates range from 5 to 20%. Prevalence estimates are generally higher among those seeking treatment: As many as 50% of veterans seeking treatment screen positive for PTSD, though much fewer receive a PTSD diagnosis. Combat exposure is the only correlate consistently associated with PTSD. When evaluating PTSD prevalence estimates among this population, researchers and policymakers should carefully consider the method used to define PTSD and the population the study sample represents.