This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book concerns the practical knowledge of teachers. It makes the assumption that teachers hold a complex, practically-oriented set of understandings which they use actively to shape and direct the work of teaching. The book explores the notion of the teacher's 'practical knowledge' in a series of open-ended discussions with one teacher, Sarah. These discussions dealt with her involvement in the planning and development of an experimental course and with a variety of issues arising out of this work, including Sarah's attitudes to teaching and learning, her conception of her subject matter in the two areas in which she taught her values, commitments and career plans as a teacher. The book uses the term 'practical knowledge' because it focuses attention on the action and decision-oriented nature of the teacher's situation.