Criminologists are increasingly turning their attention to the many points of intersection between immigration and crime control. This book discusses the detection of unlawful non-citizens as a distinct form of policing which is impacting on a growing range of agencies and sections of society. It constitutes an important contribution not only to the literature on policing but also to the field of border control studies within criminology. Drawing on the work of Clifford Shearing, Ian Loader and P.A.J. Waddington, it offers new theoretical approaches to the study of police powers and practice.