This book examines how an exemplary ESL teacher develops expertise in ESL teaching and  identifies the distinctive features in these aspects that differentiate her from three other ESL teachers, two experienced ones and a novice teacher. Through the stories of the professional development and the vivid portrayal of these teachers in action, this book explores the nature of knowledge held by an expert teacher and why some teachers become experts while others fail to rise above mediocrity. Of considerable interest to researchers, teachers,  and teacher educators in Applied Linguistics and TESOL.