What makes a language learner an expert? What do skilled learners and second language users do as they listen, speak, read and write? This book provides an overview of how questions like these are being answered by applied linguists. It considers also similar questions about the expertise of language teaching professionals, task designers and teacher trainers as well as teachers. Throughout, the book draws heavily on the literature dealing with expertise in general, applying ideas and insights from that literature to the language learning and teaching fields. Questions about how expertise can be taught are also considered. The book will be of interest to all those involved in language teaching and language teacher education who believe that knowing what constitutes expertise in these areas is essential if we are successfully to train experts.