Severity of brain injury exists along a broad continuum (see Chapter 2, Neuropathology). Discerning the extent of injury neuropathology in humans is limited to examination of the brain either at autopsy or indirectly through neuroimaging. This has resulted in efforts to classify brain injury severity on the basis of clinical signs and symptoms evident at the time of injury or shortly thereafter. This approach does not take into account the forces that contribute to the injury, measures of associated neuropathology, or ultimate functional outcome. Thus, these efforts are at best an approximation of injury severity. Nevertheless, over the last several decades it has become the convention to categorize injury severity into three groups: mild, moderate, and severe, based on initial presentation. At the more severe end of the injury spectrum, the correlation between initial injury severity rating and various outcome measures is relatively robust (Rassovsky et al. 2006). At the milder end of the spectrum, this correlation is less tight, and over the last 100 years (Evans 1994) this has generated confusion with regard to the typical presentation, trajectory of recovery, and outcome of milder injuries. Interest in this topic has grown dramatically since the last edition of this book—about one-fifth of the papers published on mild traumatic brain injury (mTBI) in the past 60 years appeared in the last 5 years. This chapter presents an overview of issues pertinent to mild brain injury.