In this study, the authors were interested to investigate how a lesson plan study (LPS) activity conducted within the context of a computer-supported collaborative learning (CSCL) environment would impact on prospective teachers’ repertoires of pedagogical-content knowledge (PCK) about teaching primary school mathematics. By the end of this study, it was found that the prospective teachers had made considerable advances to their repertoires of PCK. The authors found several factors that influenced growth in PCK.