The authors examine the relationship of the two signature injuries experienced by military personnel serving in Afghanistan and Iraq: posttraumatic stress disorder (PTSD) and mild traumatic brain injury (mild TBI). Studies show that a substantial minority of those serving develop persistent emotional sequelae (such as PTSD and other psychological health problems) and/or somatic or cognitive sequelae (postconcussive symptoms) of traumatic exposure. Remarkably, the mechanism (emotional versus biomechanical) and locus (head versus other regions) of injury are weak determinants of whether an individual develops PTSD, persistent postconcussive symptoms, or both. Preexisting or traumatically acquired cognitive dysfunction can increase the risk for these syndromes, probably by reducing cognitive reserve. Structural and functional neuroimaging studies can be interpreted to explain part of the shared symptomatic and functional variance in these syndromes, but this literature is far from consistent and serves mainly to raise new, challenging questions about mutual pathophysiology. The frequent confluence of PTSD and persistent postconcussive symptoms in military personnel strains the bounds of these constructs. New studies are needed to improve our understanding of how emotional and biomechanical stressors can yield these adverse outcomes and how such outcomes can be prevented and treated.