In addition to cross-cultural and environmental stressors, aid workers and missionaries are frequently exposed to trauma. We explored the frequency of traumatic events, their mental health impact, and factors associated with posttraumatic stress in two groups of missionaries, one representing a predominantly stable setting (Europe) and the other an unstable setting (West Africa). The 256 participants completed self-report measures assessing lifetime traumatic events, current posttraumatic stress, depressive and anxiety symptoms, resilience, and functioning. The rate of traumatic events was significantly higher in the unstable setting. More-frequent traumatic events were associated with higher posttraumatic stress. Factors associated with the severity of posttraumatic stress were depression, functional impairment, subjective severity and number of traumatic events, and the level of resilience.