This book offers essential insights into the challenges and complexities surrounding the medium of instruction (MOI), its impact on all languages and stakeholders in multilingual contexts, educational processes, developments and outcomes. MOI has been a prominent topic in recent debates on the role of languages in education in multilingual contexts, partly because prioritizing one language over others as the medium of instruction has a profound impact on all languages and stakeholders in multilingual contexts. These include, to name but a few, (language) teachers, teacher educators, students, and policymakers, as well as industries and enterprises built around the needs and expectations of these stakeholders. This book presents high-quality empirical research on education in multilingual societies. It highlights research findings that, in addition to providing descriptions of language learning, development and use in language contact and multilingual contexts, will help shape future language education policy and practices in multilingual societies.