Amy Tsui has provided a major service to the field of applied linguistics in this first detailed study of what expertise in language teaching consists of and how it develops in language teachers. Through exploring the classroom practices and knowledge of her subjects, in four illuminating case studies, Dr. Tsui succeeds in clarifying the nature of expertise in language teaching, the factors that shape and influence its development, and how teachers employ their expertise in teaching. In the process, the author critically examines an extensive literature on teacher cognition and shows how teachers’ theories, knowledge, experience and goals shape their classroom practices and their ability to move from novice to expert. Understanding Expertise in Teaching is a valuable addition to the literature on teacher cognition in language teaching and will be of considerable interest to researchers, teachers, and teacher educators in Applied Linguistics and TESOL.