Primary teachers generally have a deep knowledge of their students and a broad repertoire of pedagogical practices, particularly practices that involve teacher–student and student–student interactions, but they often lack in-depth content knowledge and have a limited repertoire of subject-specific pedagogies. This is almost always a particular issue for their teaching of science. Pedagogically appropriate teacher engagement of/with students requires teachers to have both appropriate content knowledge and knowledge of their students. In this context the idea of “knowledge in action” is central. In-depth content knowledge and subject-specific pedagogies influence teachers’ pedagogical content knowledge. Effective formative assessment or Assessment for Learning (AfL) interactions mean that teachers need rich and flexible PCK in order to undertake effective interactions with diverse groups of students. In this chapter we highlight what we mean by effective AfL interactions, teachers’ knowledges required for effective interactions and how we might enhance teachers’ knowledges to enhance the potential for student learning.