Posttraumatic stress disorder (PTSD) and complex PTSD (CPTSD) are often chronic and debilitating conditions that can cause significant impairment in an individual’s personal, professional, and social life. However, research pertaining to posttraumatic stress responses among older adults is lacking. The present thesis aimed to address this gap in the literature by evaluating the structure, psychiatric comorbidity, and correlates of posttraumatic stress responses among older adults. The most contemporary models of PTSD (as outlined in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders [DSM-5] and the eleventh version of the International Classification of Diseases [ICD-11]) provided a valid representation of PTSD among older adults. Four DSM-5 symptoms demonstrated sex item-bias, with females being more likely to endorse three symptoms (B1: 'unwanted memories', B4: 'feeling upset', and E6: 'sleep problems') and males being more likely to endorse one symptom (E2: ‘reckless or self-destructive behaviour’). Alternatively, there was no evidence of item-bias for the six ICD-11 symptoms. Regarding comorbidity, a dimensional framework of psychopathology was successful in accounting for the psychiatric comorbidities of PTSD. More specifically, evidence supported the existence of two distinct subtypes of ICD-11 PTSD psychiatric comorbidity classes among older adults, with the higher comorbidity class being associated with a history of attempted suicide. In terms of PTSD/CPTSD correlates, evidence of longitudinal relationships between subtypes of loneliness (social and emotional) and PTSD, and cross-sectional relationships between loneliness and CPTSD symptoms were found in older adults. These results have important implications for both research and clinical practice regarding the assessment and treatment of PTSD and CPTSD in later life. These findings imply that the current diagnostic models of PTSD are applicable in older adults; identify the psychiatric disorders that are likely to manifest alongside PTSD in later life; and highlight loneliness as a clinically meaningful construct among trauma-exposed older adults.