There has been a tendency in the more recent revisions of the Diagnostic and Statistical Manual (DSM) of the American Psychiatric Association (APA) and in studies of post-traumatic stress disorder (PTSD) to expand somewhat haphazardly the DSM-III formulation of Criterion A traumatic events. This chapter explains how the field might develop a more rigorous Criterion A definition. The definition of traumatic stressors was broadened in DSM-IV. The strongest claim to primacy would be for stressors that are necessary and sufficient to elicit the syndrome of PTSD symptoms and signs. Stressful events occur along a continuum of situations in which people live their lives. What to include in and exclude from the definition of Criterion A stressors in the next formulation of PTSD should depend on a great deal of systematic research with all types of situations and related negative events.