The purpose of this study was to explore the effect of Content Representations (CoRes) construction, and reflective peer discussions on pre-service physics teachers' pedagogical content knowledge (PCK). Participants consisted of 16 third year pre-service physics teachers; 12 females and 4 males. The results show that the majority of participants made positive improvements to their initial PCK. Participants became more knowledgeable about students' misconceptions, developed improved orientations to teaching, and suggested more responsive instructional strategies and assessment strategies along with more elaborate justifications. Discussion focuses on implications of these results for professional development of pre-service science teachers and research on PCK.