Today, local police are being integrated into federal immigration enforcement on a scale never seen before in American history. This transformation of immigration law is not the result of the high-profile efforts by Arizona and a few other states to regulate migrants. Instead, it is the product of a largely overlooked federal program known as “Secure Communities.” Launched three years ago, the program’s goal is simple: to check the immigration status of every single person arrested by local police anywhere in the country.

Secure Communities represents the future of immigration enforcement. It dramatically lowers the information cost of identifying immigration violators, accelerates the ongoing convergence of the immigration and criminal bureaucracies in the United States, and reshapes the structure of immigration federalism. Despite its significance, however, little is known about the program.

This Article, part of a larger project providing the first large-scale empirical evaluation of Secure Communities, uses the program’s rollout to explore a pervasive feature of criminal and administrative law that rarely lends itself to empirical examination—the role of discretion in policing. The breadth of discretion wielded by police and prosecutors is probably the single most important feature of modern law enforcement. Controlling that discretion—through judicial intervention, administrative design, and so on—has consequently become the central preoccupation of criminal and administrative law scholarship. For all that attention, however, we often have little sense of how law enforcement officials actually wield the discretion they possess. Anecdotal accounts abound, but systemic empirical evidence is rarely available. This is even truer with respect to immigration enforcement, which represents one of the largest and least studied law enforcement bureaucracies in the United States.