Much attention has been focused on how states produce knowledge about the people they govern; far less has been written about those aspects of society that states choose to keep obscure. This book makes an original contribution to understanding state ignorance by focusing on one of the most complex and contested social issues of our day: the governance of irregular migrants. Tracing the evolution of state monitoring and control of irregular migrants from the 1960s to the present day across France, Germany and the United Kingdom, the authors develop a theory of 'state ignorance', setting out three complementary ways of understanding such oversights: ignorance as omission, ignorance as strategy, and ignorance as ascription. The findings upend dominant approaches, which tend to assume that states are preoccupied with producing knowledge about their populations, and argues that states have actually been keen to sustain ignorance about their unauthorised populations.