This essay suggests that 9-11 constitutes a critical event for urban scholars because it encapsulated and catalyzed a trend toward urban-based terror that had been building for a decade or more. The best way to put 9-11 in perspective and understand its significance is through a paradigm. In constructing the 9-11 paradigm, the author suggests a set of components that set the parameters for explanation. These consist of the diffusion of terror as an urban phenomenon, the economic ramifications of urban terror, and the impact of terror on the use of urban space. These components are not just mutually compatible but integral to the 9-11 paradigm. As a result of 9-11, public security, order, and protection have become central issues for cities. The paradigm also underscores the stakes held by national and intergovernmental elites in cities confronted by crisis.