This volume is the first handbook dedicated to language attrition, the study of how a speaker’s language may be affected by cross-linguistic interference and non-use. The effects of language attrition can be felt in all aspects of language knowledge, processing, and production, and can offer unique insights into the mind of bilingual language users. In this book, international experts in the field explore a comprehensive range of topics in language attrition, examining its theoretical implications, psycho- and neurolinguistic approaches, linguistic and extralinguistic factors, second language (L2) attrition, and heritage languages. The chapters summarize current research and draw on insights from related fields such as child language development, language contact, language change, pathological developments, and second language acquisition.