Irregular migration has become one of the most salient issues in European politics. Yet while extensive attention has been devoted to unauthorised entry, the issue of unauthorised residents living and working in European countries has been relatively overlooked. This neglect in part reflects the invisibility of this population, but also reveals a profound ambivalence at the heart of migration control, where states are constantly pulled by competing restrictive and permissive imperatives. We seek to understand this ambivalence, by interrogating how and why states have monitored – or overlooked – unauthorised migrants. This project engages in extensive archival and interview research that covers policy development in an unusually long period, the 1960s to the present day. This research has become especially relevant to explaining the development of the UK’s “Hostile Environment” strategy and its consequences for irregular and authorised immigrants alike, and has been consulted by government to draw lessons from the Windrush scandal.