Head injuries are common in players of these sports, and several athletes' careers have ended because of chronic neurological or psychiatric symptoms.6 An objective test to determine whether an athlete can safely return to their sport would, therefore, be highly desirable, and would reduce the current overreliance on CT scans (and the associated exposure to ionizing radiation) for this purpose Another group of individuals at risk of brain injury is military personnel, who might be exposed to several types of brain trauma in the battlefield.7 In addition to biomarkers for use in the acute and subacute phases of mild TBI, development of biomarkers that will enable clinical studies of the potential neuropathological cascades in the chronic phase of mild TBI is also important. This statement is valid not only for fluid biomarkers but also for imaging and other markers.In this Review, we provide an overview of the current research on fluid biomarkers of mild TBI. We describe the biomark