Published in COMPAS Working Paper Series (n.78, 2010), this background paper offers a critical review of key terms, concepts and evidence which will inform our ongoing qualitative study on the situation of undocumented migrant minors in the UK. The paper first addresses issues related to the definition of the target group, considering in particular the dichotomy legal/illegal immigration and showing how it fails to acknowledge two important aspects: the layered nature of legal status and entitlements, and the mobility between different statuses over time. It then introduces the debate on children in migration and illustrates some of the tensions that the migration of children produces, both discursively and in policy terms. It goes on to consider the legal and policy context in which children and families without legal status are embedded in Britain. It discusses the complex and contradictory position of this group as revealed in policy documents and existing immigration and child-related legislation. It focuses in particular on issues such as access to health and education services, and employment of undocumented migrants under 18. Finally the paper outlines the main trends in the migration of children, providing a preliminary mapping of the numbers and locations of undocumented children in Britain.