This book showcases how language learner agency can be understood and researched from varying perspectives by providing, for the first time, a collection of diverse approaches in one volume. The volume is organised into three main sections: the first section offers an introduction to the theoretical background of agency as a construct; the second section presents analyses of agency in a variety of empirical studies; and the third section focuses on the pedagogical implications of data-based studies of agency. The volume includes the work of researchers working in languages including English (ESL and EFL), Greek, Spanish, Swedish, Italian, Hindi, Marathi, Gujarati and Truku (an indigenous language in Taiwan) and with both child and adult language learners. This collection will serve as a key reference for researchers of language learning and teaching, sociolinguistics and language and identity.