This introductory text brings together diverse perspectives and research streams on language attrition -- forgetting or loss of language proficiency that can take place in one's first or additional language, for different reasons, and at different life stages. Offering a comprehensive state-of-the-art overview, Michael T. Putnam and David Natvig define key concepts and the contemporary debates in the field, discussing a variety of structural/grammatical, societal, and psychological/cognitive aspects of this phenomenon and their implications for linguistic theory and language learning. Setting the future research agenda on this subject and with helpful features like discussion questions, this will be an invaluable resource for advanced students and researchers of theoretical and applied linguistics, bilingualism and heritage linguistics, psycholinguistics, sociolinguistics, education, and beyond.