Post-traumatic stress disorder (PTSD) is a complex and chronic disorder that causes substantial distress and interferes with social and educational functioning. Consequently, identifying the risk factors that make a child more likely to experience traumatic distress is of academic, clinical and social importance. This meta-analysis estimated the population effect sizes of 25 potential risk factors for PTSD in children and adolescents aged 6-18. years across 64 studies (N=32,238). Medium to large effect sizes were shown for many factors relating to subjective experience of the event and post-trauma variables (low social support, peri-trauma fear, perceived life threat, social withdrawal, comorbid psychological problem, poor family functioning, distraction, PTSD at time 1, and thought suppression); whereas pre-trauma variables and more objective measures of the assumed severity of the event generated small to medium effect sizes. This indicates that subjective peri-trauma factors and post-event factors are likely to have a major role in determining whether a child develops PTSD following exposure to a traumatic event. Such factors could potentially be assessed following a potentially traumatic event in order to screen for those most vulnerable to developing PTSD and target treatment efforts accordingly. The findings support the cognitive model of PTSD as a way of understanding its development and guiding interventions to reduce symptoms.