Medium of instruction policies in education have considerable impact not only on the school performance of students and the daily work of teachers, but also on various forms of social and economic (in)equality. In many multiethnic and multilingual countries, the choice of a language for the medium of instruction in state educational systems raises a fundamental and complex educational question: what combination of instruction in students' native language(s) and in a second language of wider communication will ensure that students gain both effective subject-content education, as well as the second-language skills necessary for higher education and employment? Beyond this educational issue of choice of language(s) of instruction, medium of instruction policies are also linked to a range of important socio-political issues, including globalization, migration, labour policy, elite competition, and the distribution of economic resources and political power. The contributors to this volume examine the tension between the educational agendas and other social and political agendas underlying medium of instruction policies in different countries around the world, and unravel the connections between these policies and the related, critically important educational, social, political and economic issues.