There has been much research focused on the risk factors, etiology, and treatment of Post-Traumatic Stress Disorder (PTSD), since its original conceptualization as “shell shock”. The focus of this chapter is to review background literature on PTSD and trauma-focused treatments and examine the effectiveness of a group-based Cognitive Processing Therapy for PTSD in a Canadian military and police population. In this chapter, we seek to assist in bridging the gap between research and practice by investigating the effectiveness of group-based Cognitive Processing Therapy for PTSD in the context of routine mental health care. The present study is framed within concepts of psychological trauma and psychotherapy and existing literature on PTSD particularly in the context of the military and police. Reflections and recommendations are presented, marrying the results of this study with previous literature, noting the implications within the context of the greater body of research on Cognitive Processing Therapy and PTSD treatment more generally. We end with suggested implications for administration of Cognitive Processing Therapy in routine practice, cost-effectiveness of trauma-focused psychotherapy, and health care policy.