This book responds to the growing interest in the scholarship of mathematics teaching; over the last 20 years the importance of teachers' knowledge for effective teaching has been internationally recognised. For many mathematics teachers, the critical link between practice and knowledge is implied rather than explicitly understood or expressed. This means it can be difficult to assess and thus develop teachers' professional knowledge. The present book is based on two studies investigating exactly how teachers developed their pedagogical knowledge in mathematics from different sources. It describes: how teachers' own teaching experience and reflection, and their daily exchanges with colleagues, are the most important sources of knowledge, how important in-service training and organized professional activities are, how teachers' previous experiences as students, their pre-service training and their reading of professional literature have less influence on their professional knowledge. The findings in this book have significant implications for teachers, teacher educators, school administrators and educational researchers, as well as policy-makers and school practitioners worldwide.