What does it mean to be an expert primary science practitioner? How do primary teachers use science subject knowledge in their practice? This book addresses these questions from a sociocultural perspective, challenging currently influential constructivist accounts. It treats the nature of teacher expertise as a dynamic capacity exemplified by those who are recognised as experts in their local communities of practice. In line with this, it provides an in-depth case study of the perspective and practices of a primary science teacher who is locally and more widely recognised as an expert practitioner. One of the conclusions is that primary science expertise is eclectic in character, requiring the employment, in a flexible way, of a variety of forms of knowledge, views of learning, and teaching strategies in order to deal successfully with the contingent situations faced in the classroom. The study of expertise-in-action is particularly important at a time when teaching is increasingly configured in terms of competencies and standards. Its implications for the education of primary science practitioners are profound. Students on education courses, teachers, and researchers will find this book of value for its careful exploration of arguments about the nature of knowledge and learning, and how these are implicated in classroom practice.