There has been ongoing discussion about the knowledge that elementary school teachers need in order to teach effectively. However, the knowledge that teachers draw on during teaching has been shown to be highly complex. Shulman (1986, 1987), for instance, suggested that teachers draw on seven types of knowledge; including their knowledge of subject matter (content knowledge), general pedagogical knowledge (knowledge about how to teach), and a special form of knowledge called pedagogical content knowledge. He saw pedagogical content knowledge as content knowledge transformed by the teacher into a form that makes it understandable to students. This proposition spawned considerable research into the proposed construct. This chapter focuses on elementary school teachers’ science pedagogical content knowledge (PCK). It proposes a way of representing the development of science PCK, and how it is related to other forms of teacher’s knowledge and to science teaching practice. A model, as an aid to its conceptualization, represents diagrammatically the development of elementary science PCK. A discussion of the model and its implications follows.