This paper reports the findings of a study that sought to identify particular centrally held beliefs of secondary mathematics teachers that underpinned the establishment of classroom environments that were consistent with the principles of constructivism. The nine crucial beliefs identified were held by one or other of two teachers and emerged from teacher and student surveys, interviews with the teachers and classroom observations. As is the case with all beliefs, these beliefs were contextually bound but since the contexts in which they applied were broader than particular classrooms it is argued that they may be generalisable to other contexts and even predictive of teachers likely to create similar classroom environments.