Research into the psychological sequalae of trauma originally started out by focusing on two sex-specific trauma populations: male war veterans with “soldier's heart”, “shellshock”, “battle fatigue”, or “war neurosis” and female victims of sexual assault or domestic violence with “rape trauma syndrome” or “battered woman syndrome”. It was noted how the flashbacks and nightmares reported by rape survivors were similar to the symptoms reported by war veterans, and several researchers and clinicians started pointing out that these trauma specific syndromes might be more similar than different (Ray, 2008; Van der Kolk, 2007). Finally, it was the large number of male Vietnam veterans and the activities of feminist and student organisations, which led to the inclusion of the first PTSD diagnosis into the American DSM-III in 1980 (American Psychiatric Association, 1980). With the introduction of the PTSD diagnosis, the idea that the male war neurosis and the female rape trauma syndrome were ultimately manifestations of the same disorder was widely accepted. As a result, most research on PTSD has been based on the idea that males and females are traumatised in similar ways, and studies on sex differences in PTSD have primarily focused on examining and explaining sex differences in the prevalence and severity of PTSD, whereas studies on sex differences in the manifestation of PTSD are almost completely absent from this otherwise expanding area of research. Most literature on sex differences in PTSD uses the terms sex and gender interchangeably. Traditionally, however, the term sex refers to the biological distinction between males and females, whereas gender refers to the much more complex cultural understanding of masculine and feminine gender roles as they are viewed in the context of not only sex, but also culture, subculture, age, race, class, and sexual orientation. Even though many studies on PTSD claim to examine gender differences, most studies have in fact studied sex differences and only few have looked into the effect of masculinity or femininity on PTSD. Although this chapter will focus primarily on sex, we acknowledge that gender is likely to affect the development and maintenance of PTSD in males and females as well. The contribution of sex versus gender based explanations for sex differences in PTSD will be discussed throughout the chapter, although the topic merits a more thorough discussion than is possible here.