This book focuses on the migration of undocumented minors arriving recently to the United States and the European Union, flows that are often labeled ‘undocumented’, ‘illegal’, or ‘irregular’ and due to their sudden increase, they have been described in the media, policy circles, and scholarly work as a ‘surge’ or a ‘crisis’. Leading scholars examine the intricacies of the contexts that these minors encounter in the localities where they arrive, including the legal and ethical frameworks for protecting unaccompanied minors, governmental decisions about the ‘best interests’ of the children, these minors’ expressions of their own best interests or agency as they navigate immigration and social service systems, conditions in detention centers, and the health and social service needs in receiving communities. Though definitions and techniques for counting unaccompanied migrant minors differ between the U.S. and the EU, this book underscores the immigrant minors’ common vulnerabilities and strategies they adopt to protect themselves and improve their circumstances. At the same time, contributors to the volume highlight common challenges that both European and U.S. governments face as they develop policy strategies and legal mechanisms to attempt to balance the best interests of these children with national interests of the countries in which they settle. The chapters in this book were originally published as a special issue of the Journal of Ethnic and Migration Studies.