According to existing data, more than 1.5 million people experience a traumatic brain injury (TBI) each year in the United States. Of them, as many as 75 percent sustain a mild traumatic brain injury-or MTBI. These injuries may cause long-term or permanent impairments and disabilities. Many people with MTBI have difficulty returning to rou- tine, daily activities and may be unable to return to work for many weeks or months. In addition to the human toll of these injuries, MTBI costs the nation nearly $17 billion each year.