This book provides a detailed overview of research on mutual intelligibility between closely related languages. The book is organized around three sections which explore different facets of mutual intelligibility research. The first section outlines how to measure levels of intelligibility and its linguistic and extra-linguistic determinants. The second part grapples with questions and issues which arise once the measuring tools are established. A final section reflects on the practical and theoretical value of studying mutual intelligibility, including issues related to language planning and policy, such as cultural, communicative, educational, and economical matters.