Post-traumatic stress disorder (PTSD) is classified as an anxiety disorder in the Diagnostic and Statistical Manual IV of the American Psychiatric Association. It is characterised by three main symptom clusters: re-experiencing (of which flashbacks are the hallmark symptom); hyperarousal; and avoidance. Diagnosis requires not only the occurrence of a traumatic event but also an intense emotional (fear, horror, or helplessness) reaction to it. Epidemiological data suggest that 80% of people will experience at least one qualifying event in their lifetime. However, prevalence rates of PTSD are much lower. Additionally, individuals with PTSD tend to experience flashbacks of only two or three particular ‘hot-spots’ of the entire trauma. Therefore, the question arises: why do some moments of trauma flash back and not others? Clinical-cognitive theories of PTSD suggest that shifts in information processing at the time of the trauma (i.e. peri-traumatically) are the mechanism whereby flashbacks are created. However, for ethical and practical reasons peri-traumatic processes in real trauma are seldom studied. An analogue traumatic event has been developed to help study peri-traumatic processes – the trauma film paradigm. This paradigm is used through-out this thesis with the goal of investigating peri-traumatic cognitive mechanisms in flashback formation. Studies 1 and 2 extend previous work using dual tasks to manipulate intrusions in-line with clinical-cognitive theories. Studies 3 and 4 use neuroimaging techniques to investigate brain regions involved in real-time peri-traumatic encoding of analogue flashbacks. Chapter 9 presents heart rate data relating to peri-traumatic physiological response to flashback encoding. All of these studies support the notion that peri-traumatic shifts in processing are involved in flashback formation. In particular, Study 3 suggests that there may be a particular neural signature associated with the formation of flashbacks. Investigation of these brain areas may help solve the questions of why some individuals are more vulnerable to PTSD and why only a few specific moments of a trauma subsequently flashback. Furthermore, an improved understanding of the cognitive mechanisms involved in flashback formation may allow theory and evidence led improvements in PTSD treatments.