There is a large body of literature on the psychological consequences of trauma experienced by individuals, but there are few studies of the acute and long-term effects of mass trauma on victimized communities. Acute stress reactions are expected, and overall resilience in the aftermath of major disasters is the rule rather than the exception. However, the available literature on mass trauma suggests that certain factors may provide clues to identifying persons at greater risk for posttraumatic stress disorder (PTSD). The severity of the trauma and the accessibility of support systems may affect long-term outcome. In industrialized countries, mass violence caused by malicious human intent may be a more virulent precursor to PTSD than other types of mass trauma, such as technological or natural disasters. School-aged children, women, persons with existing psychiatric illness, those who experienced significant losses or threat to life, those who have insufficient psychological and social support systems, and persons who exhibit symptoms of functional impairment may be at greater risk for PTSD. The findings of a population study of 2 traumatized communities are discussed. Early intervention in communities suffering mass trauma should consist of general support and bolstering of the recovery environment rather than psychological treatment; some forms of early psychological interventions may worsen outcome. There is a great unmet need for treatment and intervention guidelines for victims of mass trauma, and well-designed studies are warranted.