Traumatic brain injury outcomes can be classified as acute or chronic. Acute outcomes refer to injuries that occur immediately at the time of the injury and subsequent short-term consequences. Chronic outcomes refer to adverse outcomes that are more long-term. In mild traumatic brain injury, recovery from acute outcomes typically occurs very rapidly, i.e., within 2 weeks, with full recovery expected by 90 days. However, some 10%–15% individuals can remain symptomatic for much longer with an outcome termed post-concussive syndrome. This outcome is difficult to predict since there are very few rigorous, prospective studies of this syndrome.