An understanding of PTSD and stress-related conditions is in its infancy. This is not surprising given the fact PTSD was not recognized as a distinct diagnostic entity until 1980. Since that time, the diagnostic classification has undergone continuous change as our understanding of PTSD is refined. The authors believe that PTSD can be best understood through a dimensional conceptualization viewed along at least three spectra: (1) symptom severity, (2) the nature of the stressor, and (3) responses to trauma. Along the severity spectrum, studies that review diagnostic thresholds reveal significant prevalence of PTSD symptoms and impairment that results from subthreshold conditions. Comorbidity patterns suggest that when PTSD is associated with other psychiatric illness, diagnosis is more difficult and the overall severity of PTSD is considerably greater. With regard to a stressor criteria spectrum, the diagnostic nomenclature initially only recognized severe forms of trauma personally experienced. More recently, however, the person's subjective response and events occurring to loved ones were included. This has greatly broadened the stressor criteria by leading to an appreciation of the range of precipitating stressors and the potential impact of "low-magnitude" events. Given that responses to trauma vary considerably, another possible spectrum includes trauma-related conditions. Traumatic grief, somatization, acute stress disorder and dissociation, personality disorders, depressive disorders, and other anxiety disorders all have significant associations with PTSD. Further research is needed to clarify and expand the current understanding of PTSD and other trauma-related conditions. Consideration of the severity of symptoms and the range of stressors coupled with the various disorders precipitated by trauma should greatly influence scientific research. The future undoubtedly will bring a refinement of the current understanding of PTSD and improved treatments.