This practical resource sheds light on the role personality factors play in the genesis and treatment of Posttraumatic Stress Disorder (PTSD). Using Theodore Millon's personality-guided psychology as a framework, the authors provide insight into this challenging disorder. They discuss factors that can help protect against stress or increase vulnerability to stress. They also explore the biological foundations of PTSD and offer practical guidance on how to assess PTSD and how to incorporate an understanding of personality in the formation of the therapeutic alliance.