In the wake of the September 11 attacks, undercover policing has become an increasingly important law enforcement tool in the United States and in Europe. More frequent deployment of covert tactics has confronted democratic governments with difficult questions about these extraordinary operations should be controlled and conceptualized. How ubiquitous should covert tactics become, and how should regulatory systems respond to their increased importance? What are the challenges of taming the constantly changing and highly contested practices of undercover policing, which stubbornly resist oversight? Legal systems differ in their concerns about undercover surveillance and in their willingness to deploy covert agents and informants against a spectrum of perceived threats ranging from national security dangers like terrorism or political and religious extremism to organized crime, drug trafficking, and more ordinary forms of criminality. In most democracies, political elites, legal actors, and critics agree that undercover investigations are in some sense a evil. But national legal systems vary in what they mean by that. They have disparate conceptions of what makes covert investigations troublesome; of the proper goals of infiltration; and of the mechanisms by which undercover tactics should be legitimated and controlled. In short, legal systems forge different regulatory compromises and accord different degrees of legitimacy to the necessary evil of covert operations. Much of the scholarship about undercover policing in Europe has focused on doctrine. My study examines undercover policing empirically, through 89 qualitative, open-ended field interviews that I conducted with state and federal police officials, undercover agents, training and supervisory officials, control officers, prosecutors, and judges in fifteen of the sixteen German states. Through these interviews, I examined the ground-level strategies and practices of those who conduct, supervise, and evaluate covert operations. My empirical study examines German undercover policing as both a topic in its own right and as a contrast case that helps identify distinctive features of the American system of covert operations. The systems under comparison can, in Clifford Geertz's words, form a kind of commentary on the other's character. Each can suggest what is important and troubling about the other. Each can highlight features of the other that would seem less noteworthy if examined in isolation, without the benefit of comparison.