This paper discusses posttraumatic stress disorder's (PTSD) traumatic stressor criterion (Criterion A) in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). The history of the stressor criterion is detailed, including how it has changed over time in successive versions of the DSM. We discuss controversy over the stressor criterion, regarding arguments about whether it is too conservative or too liberal. Studies comparing Criterion A and non-Criterion A events in their association with PTSD are discussed, including the finding across studies that non-Criterion A events are just as (or more) likely to result in PTSD. Potential explanations to account for this finding are discussed, including presentation of solutions to Criterion A's limitations. Finally, legal implications for Criterion A in evaluating individuals presenting with PTSD in civil and criminal cases are discussed.