Post-traumatic amnesia (PTA) is a transient sequela of closed head injury (CHI). The term PTA has been in clinical use for over half a century, and generally refers to the subacute phase of recovery immediately after unconsciousness following CHI. The duration of PTA predicts functional outcome after CHI, but its pathophysiological mechanism is not known. This paper compares current methods of determining the duration of PTA, summarizes reports on neuropsychological deficits in PTA, reviews available data that allow inferences about its mechanism, and suggests methods for further exploration of its pathophysiology.