Post-traumatic stress disorder (PTSD) is a debilitating condition that may arise after exposure to shocking, frightening, or dangerous events. Hallmark symptoms are re-experiencing, avoidance, and hyperarousal. Other common symptoms are more ancillary and overlap with other psychiatric disorders (e.g., anhedonia, interpersonal problems, and affective dysregulation). The variety of symptoms associated with PTSD allows for large differences in symptom presentation between individuals. Studies of the latent structure of PTSD (e.g., latent class analysis, confirmatory factor analysis) have been highly influential in the conceptualisation of the disorder. The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) and the eleventh edition of the International Classification of Diseases (ICD-11) have taken vastly different approaches to handling the symptom variety, with DSM-5 encompassing a broad definition, and the ICD-11 instead proposing a narrow PTSD construct and introducing the new diagnosis complex PTSD (CPTSD), comprising PTSD in conjunction with ancillary symptoms. The principal aims of the present thesis were to examine how different symptom presentations of PTSD were associated with well-known predictors of PTSD and prospective outcome, to evaluate the dimensional structure of PTSD as it is proposed in current diagnostic nomenclature, to provide methods for assessing PTSD in the Swedish language, and to evaluate the diagnostic agreement between DSM-5 and ICD-11. Using latent class analysis, subgroups with differences in PTSD symptom presentation were examined and assessed regarding their predictive validity. In a sample of natural disaster survivors, subgroups differed mainly in symptom severity. In a mixed trauma sample, subgroups differed in their likelihood of fulfilling hallmark versus ancillary symptoms, and in self-reported concurrent and prospective psychological distress. As for the dimensional structure of DSM-5 symptomology, support was not found for the four-factor DSM-5 model, but rather for a six-factor and a seven-factor model. For ICD-11 symptomatology, the ICD-11 model was supported, both with and without a higher-order separation of PTSD and CPTSD. Two instruments for assessing PTSD were evaluated: the PTSD checklist for DSM-5 (PCL-5) and the International Trauma Interview for ICD-11 (ITI). Results indicated support for both instruments as valid and reliable tools. The diagnostic agreement between DSM-5 and ICD-11 was moderate. Summarised, the studies suggest that variables such as secondary stressors and event-specific exposure influence symptom expression, and that the combination of hallmark and ancillary symptoms of PTSD is associated with the long-term maintenance of psychological distress. Results support the use of the PCL-5 and the ITI as assessment tools for DSM-5 and ICD-11 PTSD. The insufficient agreement between DSM-5 and ICD-11 PTSD and CPTSD poses a challenge for future researchers and clinicians
