The scarcity of significant terrorist attacks in recent years has led Americans to assume that the days of mass casualty attacks are in the past. But history teaches us to beware of the assumption that recent trends foretell the future. Americans are paying insufficient attention to unexpected events in which terrorists inflict serious harm on the United States. The routine occurrence of unpredictable events offers good reason to invest broadly in counterterrorism and homeland security. At the same time, several potential disasters are already discernible.