The Education Faculties within the universities are looking to meet the needs of their learners more completely. Teaching and learning strategies are being adopted to make educational provision more flexible by using computer supported collaborative learning environments. However, research has shown that in preservice teacher programs, many teachers do not have a complete knowledge of subject-matter or pedagogical content knowledge. This study investigated how a lesson-planning task within the context of a computer supported collaborative learning environment can facilitate the growth of teacher knowledge, specifically subject-matter knowledge and pedagogical content knowledge about the teaching of ratio and fraction. The findings from this research showed that using a lesson-planning task facilitated the development of teacher knowledge, specifically subject-matter knowledge and pedagogical content knowledge. Within the computer supported collaborative learning community, the preservice teachers were able to articulate their current understandings and re-define their knowledge of fraction and ratio. The thesis concludes with a conceptual framework and some specific guidelines for the design of a scaffolded computer-mediated community and discusses implications for the design of future collaborative learning environments.