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Background: The mechanisms by which trauma increases the risk of psychosis are unclear. This thesis aimed to investigate whether post-traumatic stress disorder (PTSD) mediates the relationship between trauma and psychosis, and to examine the feasibility of conducting a large randomised controlled trial (RCT) to evaluate the effectiveness of a trauma-focused therapy (Eye-Movement Desensitization and Reprocessing (EMDR)) on preventing psychosis in people with an at-risk mental state (ARMS). Methods: Two studies were conducted. The first examined whether PTSD mediates the relationship between trauma and psychosis using data from a longitudinal birth cohort. The second was a feasibility study. 14 participants were recruited from the Early Intervention (EI) teams. Interviews with nine patients and three therapists explored their views and experiences of EMDR. Interviews were also held with 11 patients and 41 clinicians from primary and secondary care services to understand ARMS patients' pathways into care. Results: PTSD symptoms mediated ~14% of the relationship between childhood trauma and adolescent psychotic experiences, and ~8% of that between adolescent trauma and PTSD in early adulthood. The feasibility study found that it would be difficult to recruit sufficient ARMS patients for a large RCT from the EI teams. The challenge was due to the small number of ARMS patients in the EI teams. The qualitative work suggested this was because of primary care clinicians not identifying these patients, high thresholds for accessing specialised services, and limited treatment availability. Patients' and therapists' accounts of EMDR suggested patients who completed therapy benefited, although therapists differed in their views regarding the effectiveness of EMDR. Conclusion: PTSD plays an important role in the development of psychosis. Interventions to target trauma processing may offer an opportunity to intervene to prevent psychosis. Trials recruiting ARMS patients may struggle to recruit unless they extend recruitment to primary care services.