Young children start school already able to do a surprising amount of mathematics. This book examines the nature and origin of subject knowledge and is based on information gathered from observing the interactions between teachers and their first-year pupils. It demonstrates the necessity of the classroom teacher to draw on many kinds of knowledge in order to deal with various issues surrounding classroom learning and teaching. Two important core areas are knowledge of lesson structure and of subject matter; this book address the area of subject matter and, as such, it should be of interest to classroom teachers and lecturers in education.