This paper reports on teachers? practical knowledge [PK] about peer interactions [PI] in learning mathematics. The focus is on high school teachers who consistently engaged students PI in their teaching. Data consisted of interviews and classroom observations. Findings indicate that these teachers have PK of students? roles in PI and learning activities and teacher?s behaviors to support PI that creates a meaningful classroom culture to facilitate PI in learning mathematics. Their classroom experiences and their conceptions of mathematics and learning played an important role in the PK. Their PK offers insights into pedagogical strategies that can be effective in facilitating PI. PK is a basis for teachers' sense making and can play an import role in teacher education. [For complete proceedings, see ED489632.]