In the context of teacher education, it could well be suggested that assessment activities that build on formative interactions between student teachers and teacher educators might offer new windows into better understanding teaching and learning. This paper presents findings from a study into a primary science teacher education initiative that seeks to build the foundations on which 24 primary science student teachers, through the use of formative assessment of their science teaching and learning, can begin developing their pedagogical content knowledge (PCK). In the project, formative assessment consists of activities used by teacher educators to stimulate interactions, self- and peer-assessment in order to provide insights into how student teachers develop their PCK during a semester. Content Representations (CoRes), were used as a tool to unpack the student teachers' approach to teaching a science topic and the reasons for that approach. The results indicate that the use of CoRes, together with subsequent self-assessment and formative interactions with teacher educators and peers, do have the potential for PCK development for student teachers. The results further highlight the need for developing reliable and valid tools for capturing and assessing student teachers' PCK in pre-service teacher education.