As unprecedented levels of human mobility continue to define our era, criminal justice institutions in countries around the world are increasingly shaped by mass migration and its control. This collection brings together legal scholars from Europe and the United States to consider the implications of the attendant changes on the exercise of state penal power and those subject to it. The contributions in this special issue are united by a shared set of questions about the salience of citizenship for contemporary criminal justice policies and practices. They are specifically concerned with questions of fair and equal treatment, the changing configurations of state sovereignty, and the significance of migration on criminal justice policies and practices. Collectively, the articles show how, in grappling with mass mobility and diversity, states are devising novel forms of control, many of which Erode basic criminal justice principles and reinforce existing social hierarchies.