The majority of sport-related head injuries are mild traumatic brain injury (mTBI) events, traditionally referred to as concussions. Per consensus definition, "concussion is. . . a complex pathophysiological process affecting the brain, induced by traumatic biomechanical forces" (Aubry et al. 2002, p. 57). Five specific common features are noted: 1) causation by a direct blow to the head, face, neck, or elsewhere on the body with an "impulsive" force transmitted to the head; 2) a typically rapid onset of short-lived impairment of neurological function that resolves spontaneously; 3) acute clinical symptoms primarily reflecting functional disturbance versus structural injury; 4) a resulting graded set of clinical syndromes that may or may not involve loss of consciousness (LOC), with resolution of clinical and cognitive symptoms generally following a sequential course; and 5) typical association with grossly normal structural neuroimaging studies. Five domains of specific signs and symptoms of acute concussion are noted in Table 27–1; problems in one or more of these domains raise the index of suspicion for diagnosis of concussion.