Mild traumatic brain injury (TBI) is a very common injury, resulting in immediate and possible long-term symptoms. The accurate and consistent definition of mild TBI is important in the initial and rehabilitation management of the injury, and in research concerning mild TBI. A definition of mild TBI has been developed by the Head Injury Interdisciplinary Special Interest Group of the American Congress of Rehabilitation Medicine. Within the spectrum of injury severity in mild TBI there are several classification systems, primarily used in management of acute mild TBI, that breakdown mild TBI into grades of injury severity. These are based upon the presence or absence of mental status changes, amnesia, loss of consciousness, anatomical lesion or neurological deficit.