Effective postterrorism public health interventions require the recognition that behavioral consequences are, in fact, the intent of terrorists. The authors searched published and unpublished post-1980 studies that documented population-level behavioral and psychological consequences of terrorist incidents, focusing on posttraumatic stress disorder (PTSD). Results were tabulated, and random effects models were used to calculate overall effect sizes. The analysis indicates that in the year following terrorist incidents, the prevalence of PTSD in directly affected populations varies between 12% and 16%. The review also shows that this prevalence can be expected to decline 25% over the course of that year. These prevalence estimates mask great variability, depending on who is being studied, who is conducting the study, and where the event occurred. Higher rates of disease are reported for survivors and rescue workers, and higher overall rates are also reported from studies conducted in Western Europe compared with studies conducted in North America. Prior psychiatric diagnoses are strongly associated with subsequent PTSD and may be a useful triage factor, particularly when considered together with factors such as female gender and direct exposure to events as either a survivor or rescuer. The review indicates that these associations are consistent across study types and environments and represent important variables to consider when developing triage, outreach, and treatment programs.