Traumatic brain injury (TBI) occurs when external physical forces cause damage to the brain, whether from impact, penetrating objects, blast waves or rapid movement of the brain within the skull.  Currently, TBI has three severity classifications (mild, moderate, severe) that are based primarily on the Glasgow Coma Scale.  The classification of mild TBI also includes concussion, commonly defined as a transient alteration of brain function due to exposure to external physical forces. While acute TBI can be life threatening, TBI also can have long-term sequelae including cognitive and physical disability, post-concussion syndrome (PCS), and may contribute to the development of chronic traumatic encephalopathy (CTE). PCS results when various symptoms of concussion last weeks, months or more than a year following concussion.  CTE is a delayed neurodegenerative disorder that results from repetitive mild injury to the brain and can only be diagnosed postmortem. Additional general information about TBI can be found at the NINDS TBI Hope Through Research website.