Posttraumatic stress disorder (PTSD) commonly occurs with other psychiatric disorders. Data from a recent epidemiologic survey indicate that approximately 80% of individuals with PTSD meet criteria for at least one other psychiatric diagnosis. PTSD is particularly likely to be comorbid with affective disorders, other anxiety disorders, somatization, substance abuse, and dissociative disorders. Comorbidity may affect the presentation and clinical course of PTSD. Because of the relative frequency of traumatic events and the heterogeneity of presentation of PTSD, screening for traumatic events and PTSD should be standard in both psychiatric and primary care practice. Additionally, individuals with PTSD should be screened for psychiatric comorbidity. Accurate assessment of comorbidity may be important in determining optimal psychotherapeutic and pharmacotherapeutic treatment options for individuals with PTSD.