This case study investigated how an English as a Second Language (ESL) teacher's cognition has both changed and stayed constant over a period of eight years and the factors to which the teacher attributes changes or lack of changes. The study followed the teacher over the course of a 10-week period and compared videos of the participant's teaching from eight years ago to her current teaching.Interviews, observations, and stimulated recall were used to investigate development over the eight year span. It was found that the teacher did indicate several areas in which she demonstrated change: Teaching with fluidity, automaticity, and intuition; confidence; concerns; management of teaching enthusiasm and relationships with students; support and validation from colleagues; and managing the classroom for learning. She also confirmed several aspects of her teaching that have stayed relatively constant: beliefs and teaching philosophy; reflection; learning from past experiences; knowledge of lesson planning and curricular goals; and students' needs within the learning context. The teacher's development was analyzed through the lens of teacher expertise to determine to what degree the teacher's changes and lack of changes helped her develop into an expert. This study concludes that further research is needed to fully understand how teacher expertise is developed during the course of teachers' careers.ii ACKNOWLEDGMENTS