This paper sets out to explore science teachers' classroom assessment practices and outlines some of the tensions and synergies in changing assessment practices. It describes episodes from a collaborative action research project with science teachers designed to support the strengthening of classroom assessment practices--the King's Researching Expertise in Science Teaching project. This paper focuses on how the collaborative action research project functioned as a tool for generating formative feedback for the teachers and how this feedback supported the teachers in making sense of and further developing their classroom assessment practices.