In this article, we draw from research on cognitive development, sociocultural learning, and organizational development to examine the key factors in teachers' professional learning. Our review of this literature suggests that looking at teachers' prior knowledge, the nature of their interactions, the representations they produce and use, and the contexts in which they operate can all help to explain how teachers learn about good practice and the roles that informal inquiries as well as more formal research can play in the process. We apply these ideas in an analysis of the development of one teacher's key insights while teaching English Language Learners during the first four years of her career.