The diagnosis of acute stress disorder (ASD) was introduced to describe initial trauma reactions that predict chronic posttraumatic stress disorder (PTSD). This review outlines and critiques the rationales underpinning the ASD diagnosis and highlights conceptual and empirical problems inherent in this diagnosis. The authors conclude that there is little justification for the ASD diagnosis in its present form. The evidence for and against the current emphasis on peritraumatic dissociation is discussed, and the range of biological and cognitive mechanisms that potentially mediate acute trauma response are reviewed. The available evidence indicates that alternative means of conceptualizing acute trauma reactions and identifying acutely traumatized people who are at risk of developing PTSD need to be considered.