The author concludes that international and national terrorism will continue to pose a unique, pervasive, and costly challenge to the ideals, principles, and safety of the United States well into the 21st century. After tracing the roots of politically motivated violence to its earliest manifestations in ancient history, this chapter identifies the changing characteristics of modern terrorism, which have made its contemporary versions more potent, more destructive, and more lethal. The combination of rapid technological development in weaponry, telecommunications, and mobility has greatly increased the vulnerability of modern, industrialized nations. Nuclear plants, pipelines, powergrids, public transport systems, and centralized computer banks that store vital records are potential terrorist targets that could create significant losses and disruptions. The confluence of these developments means that ever- smaller groups of malcontents and extremists will be able to inflict ever greater harm and damage now and into the future. The author draws distinctions among national, international, and transnational terrorism. Study questions and 57 references