The United States is not prepared to deal with an attack by a terrorist group using an improvised nuclear device, the author says. It should get prepared, because the risk is real even if the probability is low, and doing so could save a great many lives. The author explores the potential impact of a 10–15-kt improvised nuclear device set off in New York City. The initial blast would kill between 75,000 and 100,000 people in seconds. Another 100,000–200,000 people would be injured, many of them dying within weeks or months, some with burns, others with impact injuries, and some with acute radiation syndrome. The demands on the medical system would be vast and overwhelming, all the more so because the bomb would have destroyed much of the capacity to respond. Current planning efforts are not sufficient to manage the carnage.