How does one identify a phenomenon as radically new or "unprecedented"? Do, in fact, "unprecedented" phenomena exist at all, as presumably some degree of continuity marks every state of affairs? If, however, the idea of continuity is taken too far, are we not at risk of domesticating a radical tendency by conceptually transmuting it into something that is already known? These questions are of pressing importance since September 11th, as commentators warn that America and its allies face radically new enemies in the guise of "rogue" states, movements, and terrorist organizations. Yet sociology has had to grapple with similar issues before. In the 1930s and 1940s, the consolidation of fascist and Nazi movements also taxed sociological understanding and competence to the maximum. This article describes aspects of American sociology's response to that earlier challenge, contrasting it with the approach of Hannah Arendt, who condemned the discipline for systematically failing to appreciate the uniqueness, enormity, and gravity of the events that assailed the epoch. Arendts critique of sociological methods is followed by a case study-Theodore Abel's investigation into National Socialism-that lends some credence to her misgivings. The work of other sociologists of this period-notably, Talcott Parsons's-is also briefly considered. The concluding section of the paper examines the theoretical problems involved in seeking to conceptualize an "unprecedented" event (movement, institution).