An individual with a mild traumatic brain injury (mTBI) will, by definition, have a Glasgow Coma Scale score (GCS) (Teasdale & Jennett, 1974) of 13-15 within 30 minutes of injury (i.e., typically by the time they are initially assessed by emergency medical personnel). Thus, within 30 minutes of injury, they obey commands and, at worst, may be disoriented and consequently have confused or inappropriate speech. Also by definition, disorientation, if present, lasts less than 1 day. Approximately 70-90% of patients who sustain TBIs have had a mild TBI (Cassidy et al., 2004). Mild TBI is believed to result when injury triggers a pathologic neurochemical cascade, but it is insufficient to produce widespread neuronal dysfunction or the axonal disruption that characterizes more severe brain injuries. The majority of these patients make excellent neurobehavioral recovery, but some have persistent and at times disabling symptoms (Carroll et al., 2004; see also Chapter 18 by Nicholson and Martelli). The formal criteria for mTBI have been defined by the American Congress of Rehabilitation Medicine (1993) (see Chapter 15 by Barth et al. for a discussion of these criteria).In TBI cohort-based neuroimaging studies, Borg and colleagues (2004) report that 5% of individuals who have a GCS score of 15, 20% with a GCS score of 14, and 30% with GCS score of 13 have abnormal findings on clinical computed tomography (CT). However, in those with an initial GCS of 13-15 (i.e., mTBI), if clinical neuroimaging findings are present, the classification changes to "complicated mTBI," which has a 6-month outcome more similar to moderate TBI (Williams et al., 1990). In this chapter, mTBI will be used to refer only to individuals with normal initial clinical neuroimaging findings.