This chapter illustrates the conceptual and historical background of our current understanding of post-traumatic stress disorder (PTSD) and the implication of that history for current and future research and practice. It follows two parallel paths, conceptual and chronological, showing a progression in defining, recognizing, and diagnosing PTSD, The chapter discusses a stable conceptual space within which key clinical features of PTSD are conserved across time and cultures. The chapter also considers recent conceptual and definitional challenges emanating from new computational tools and advances in neurobehavioral research.