Since 2001, the United States has created or reorganized more than two counterterrorism organizations for every terrorist arrest or apprehension it has made of people plotting to do damage within the country. Central to this massive enterprise is ''ghost-chasing,'' as less than one alarm in 10,000 is an actual threat-the rest all point to ghosts. And the vast majority of the leads deemed to be productive have led to terrorist enterprises that were either trivial or at most aspirational. As John Mueller and Mark Stewart suggest in Chasing Ghosts, this is often an exercise in dueling delusions: a Muslim hothead has delusions about changing the world by blowing something up, and the authorities have delusions that he might actually be able to overcome his patent inadequacies to do so. Mueller and Stewart systematically examine this expensive, exhausting, bewildering, chaotic, and paranoia-inducing process. They evaluate the counterterrorism efforts of the FBI, the National Security Agency, the Department of Homeland Security, and local policing agencies. In addition, applying a new set of case studies, they appraise the capacities of the terrorist ''adversary'' and discuss what they calls ''the myth of the mastermind.'' They also assess public opinion, a key driving force for counterterrorism efforts. The yearly chance an American will be killed by a terrorist within the country is about one in four million under present conditions. However, polling data suggest that, although over a trillion dollars has been spent on domestic counterterrorism since 2001, Americans continue to be alarmed and say they do not feel safer. No defense of civil liberties is likely to be terribly effective as long as officials and the population at large continue to believe that the threat from terrorism is massive, even existential.Mueller and Stewart do not argue that there is nothing for the ghost-chasers to find-the terrorist ''adversary'' is real and does exist. The question that is central to the exercise, but one the ghost-chasers never really probe, is an important and rather straight-forward one to which standard evaluative procedures can be applied: is the chase worth the effort? Or is it excessive given a serious consideration of the danger that terrorism actually presents?