This paper describes the actions of one secondary school mathematics teacher in establishing a community of mathematical practice within which students acquire not only knowledge and skills, but also the epistemological values of mathematics. Classroom observations over a period of 18 months, together with interview and questionnaire data, are used to sketch a model of the teacher's interactions with his students as he works toward creating a culture of mathematical sense-making. The results indicate four aspects of the teacher's role that are particularly important in establishing a classroom community of practice: (1) modeling mathematical thinking; (2) cognitive and social scaffolding; (3) encouraging individual reflection, self-monitoring and checking; and (4) introducing tools for mathematical communication. Contains 33 references. (DDR)