Post Traumatic Stress Disorder (PTSD) is a relatively common anxiety disorder that follows severe trauma. Current estimates place the point prevalence of PTSD in Western communities between 2 and 8 per cent. Chronic PTSD is frequently associated with significant depressive symptoms and significant rates of self harm and completed suicide. Understanding the nature of the relationship between Major Depression and PTSD is complicated by the significant overlap in clinical phenomenology and impact of co-morbidities such as substance abuse that may influence the presentation of both disorders. Improvements in the understanding of the neurobiology of PTSD itself over the last decade have failed to reveal absolute biological points of demarcation between these two disorders. The importance of understanding this differentiation better is underlined by available data demonstrating the impact of ongoing depressive symptoms on programmatic CBT treatment for chronic combat related PTSD. Current evidence relating to the effectiveness of biological and psychological treatments for depression co-morbid with PTSD would suggest that effective treatment will require examination of appropriate treatment sequencing and/or overlap