This book explores the criminalisation of irregular migration in Europe. In particular, it investigates the meaning, purpose, and consequences of criminalising unauthorised entry and stay. From a theoretical perspective, the book adds to the debate on the persistence of irregular migration, despite governments' attempts at deterring it, by taking an interdisciplinary approach that draws from international political economy and criminology. Using Italy and France as case studies, and relying on previously unreleased data and interviews, it argues that criminalisation has no effect on migratory flows, and that this is due to factors including the latter's structural determinants and the likely creation of substitution effects. Furthermore, criminalisation is found to lead to adverse consequences, including by contributing to vicious cycles of irregularity and insecurity.