Posttraumatic stress disorder (PTSD) and social anxiety disorder (SAD) demonstrate a high degree of comorbidity. Indeed, rates of past-year SAD among individuals with PTSD have been shown to range from 14.8 to 46.0% within treatment-seeking and veteran samples (Collimore, Carleton, Hofmann, & Asmundson, 2010). Although PTSD and SAD are individually associated with negative mental health outcomes, preliminary data suggests that the co-occurrence of these disorders can result in a significant increase in distress and impairment beyond the impact of either disorder alone (Zayfert, DeViva, Hofmann, 2005). Despite accumulating evidence that PTSD and SAD frequently co-occur, and growing interest into the nature and correlates of these disorders, little is known about the nature of this association. Contemporary research has largely been conducted using treatment-seeking or veteran samples which may not generalize to the population as a whole. In addition, studies of the impact of trauma type have largely focused on the impact of sexual abuse, ignoring large variability in the experience of trauma. Large-scale epidemiological studies are needed to fill existing gaps in the literature and to clarify the nature of this association within a representative sample of the general population. As such, the current dissertation examined the relationship between PTSD and SAD using Wave 2 of the National Epidemiological Survey of Alcohol and Related Conditions, a large, nationally-representative survey of American adults. A series of three thematically related studies were conducted in order to explore the nature and correlates of these frequently comorbid disorders. Study one examined patterns of trauma exposure among those with comorbid DSM-IV PTSD and SAD (PTSD-SAD). Compared to those with PTSD alone (PTSD-no SAD) or SAD alone (SAD-no PTSD), those in the comorbid PTSD-SAD group were significantly more likely to report specific types of traumas from within the assaultive violence, childhood maltreatment, or other shocking events categories. Associations between PTSD-SAD comorbidity and childhood maltreatment were significant for females only. Study two examined the impact of PTSD-SAD comorbidity on PTSD symptom expression. Compared to those with PTSD-no SAD, those with comorbid SAD demonstrated elevated rates of specific PTSD symptoms within each criterion cluster, suggesting that the presence of SAD is associated with differences in the expression of PTSD symptoms. Multiple between sex differences were noted. Finally, study three examined the impact of PTSD-SAD comorbidity on SF-12 quality of life indicators and lifetime suicide attempts. Those in the comorbid PTSD-SAD group demonstrated an elevated risk of lifetime suicide attempts and lower levels of physical and mental quality of life compared to those with PTSD-no SAD and SAD-no PTSD. Results demonstrate the adverse impact of PTSD-SAD comorbidity on psychosocial functioning and provide indicators to enhance differential diagnosis and treatment planning.