Over many centuries and especially after early 20th-century world wars, battlefield experiences brought an increasing focus on the lethal and other consequences of traumatic brain injury (TBI). World War I (WWI) and World War II (WWII) gave physicians and scientific researchers many opportunities to study the effects of TBIs on the wounded. Beginning in WWI and expanding during WWII, the U.S. military began to support physical medicine and rehabilitation (PM&R) as a medical specialty. A significant challenge for all TBI research is the heterogeneity and complexity of the TBI population. TBI research is expanding because of large-scale data sets, longitudinal studies, new laboratory biomarkers, more sophisticated neuroimaging, and advances in systems of care; continued efforts to improve research methodologies in acute care, brain injury medicine, and rehabilitation are essential for future success in identifying how to decrease morbidity and mortality and achieve the best functional outcomes.