East and Southeast Asia are home to remarkable linguistic diversity, yet this diversity is increasingly under threat as political, economic, cultural, and other social transformations reshape the region. This volume brings together two introductory chapters and ten country-focused studies—covering Korea, Japan, Taiwan, Hong Kong, Thailand, the Philippines, Singapore, and Indonesia—to identify patterns of language endangerment and marginalization, and to showcase ongoing revitalization efforts. A distinctive feature of the book is its use of Lee and Van Way’s (2016) Language Endangerment Index (LEI), which provides a common framework for balanced comparison across the communities examined.