Despite repeated calls for reliable and valid assessments of teachers’ pedagogical content knowledge (PCK), tools suitable for assessing teachers’ knowledge remain inadequate. Therefore, exploring ways of how to formatively assess student teachers ́ PCK is important; both for understanding how PCK develops and for the ways in which we might develop strategies in teacher education for enhancing science teaching and student teachers ́ professional learning. This paper is based on a science teacher education research project that aims to connect what we know from research on [formative] assessment with what we know about student teachers ́ development of PCK, and, as a consequence, seeks to determine how an articulation of this knowledge might inform meaningful approaches to science teacher education. During one semester, 24 primary science student teachers were provided with tools for assessment (Content Representation (CoRe), self-assessment and reflective activities) in order to examine their approach to the teaching of a topic and the reasons for that approach in ways that might demonstrate the extent of their PCK development over time. The project highlights the potential to positively focus on formative assessment of student teachers’ development of PCK in order to determine how that might contribute to student teachers’ professional learning.