This study examined students' understanding of, and reflective inquiry into discourse, specifically their epistemic discourse understanding and meta-discourse, and investigated their roles and relationships in fostering productive inquiry in knowledge building. The participants comprised two classes of ninth grade visual arts students inquiring into art and design. The experimental class (n = 31) engaged in knowledge building using Knowledge Forum® (KF) enriched by meta-discourse involving reflective inquiry and classroom discussion about their discourse. The comparison class (n = 32) similarly worked on KF but using regular classroom discussion. Quantitative analysis indicated that the experimental group students, who engaged in meta-discourse, showed a deeper epistemic discourse understanding and domain knowledge than the comparison students, and that epistemic discourse understanding was associated with productive KF inquiry. Qualitative analysis of the classroom meta-discourse showed that metacognitive reflection, principle-based inquiry, and idea development (i.e., meta-epistemic reflection, meta-epistemic principles, and meta-epistemic theory) support epistemic understanding and productive inquiry. We also discuss the implications of using meta-discourse to enhance epistemic discourse understanding and productive inquiry for knowledge building and computer-supported collaborative learning.