This paper explores the development of student-teachers' pedagogical content knowledge (PCK) during pre-service education. Four student-teachers in mathematics and science participated in a project teaching physics to students aged 9-11 years once a week over a 12-month period. One-third of the lessons were videotaped and the student-teachers were later interviewed using the videotape for stimulated recall. Participants reflected on their classroom practice based on their conceptual understanding of physics. This empirical study emphasises the role of teaching experience and reflection in science teacher education as a way of better understanding the complex entities that constitute a knowledge base for teaching. The paper draws attention to the value of student-teachers participating in experiences that might contribute to the development of their PCK and supports a view of PCK development as a process of transformation. (Contains 1 table.)