The study examined knowledge building and scientific understanding in the context of high school students working collaboratively on Knowledge Forum in a chemistry classroom in Hong Kong. Three specific questions were explored: 1) How can knowledge building be assessed and characterized? 2) What are the roles of knowledge building in scientific understanding? and 3) How does the community develop and how do teacher and contextual factors influence its growth and evolution? Participants included 34 Secondary Four students (Grade 10) who worked on knowledge building pedagogy mediated by a computer supported platform called Knowledge Forum for eight months. Various data including Analytic Toolkit statistics (ATK), depth of inquiry, learning portfolios and examination results were examined. Students' Knowledge Forum discourse was also analyzed quantitatively and qualitatively. The findings show that 1) a three-pronged approach using ATK, depth of inquiry, and discourse analysis was a suitable tool for tracing both individual and communal growths in knowledge work, 2) knowledge building learning portfolio and the ATK index "linked notes" predicted students' conceptual understanding, and significant differences in students' school examination, a measure of their scientific understanding, were obtained favoring Knowledge Forum students over comparison counterparts, and 3) three distinctive phases were identified suggesting students' growth in epistemic agency; as well as showing how the growth of the community was mediated by contextual and teacher factors. Discourse analysis suggests how knowledge building might be a useful means of promoting scientific understanding at both individual and communal levels interweaving progress in conceptual understanding with growth in views and ways of doing science, thus helping students to move towards both "science learning" and "learning about science". Theoretical and pedagogical significances of the findings and the limitations of this