Classrooms are crowded with people and activity, with many things happening at once. The class meets with the teacher for extended periods with events affecting subsequens actions. Many of the constraints have only limited potential for manipulation by teachers. Some popular models of change do not recognise the complexity of the classroom. Mathematics educators and curriculum developers need to acknowledge the dl,fficulty of translating theory into classroom practices which are feasible, and to identify mechanisms for overcoming inhibiting factors. To investigate the constraints on teachers, a number of primary pre-service education students were studied both in theirfinal year and in their first year of teaching. The report on one teacher is presented here. Data are presented on her background, attitudes and beliefs about mathematics teaching, and on her practice teaching experience. Similar information was collected during the beginning year, including beliefs and classroom practice. The observed teaching appeared to emphasise narrow and instrumental goals, which contradicted her stated beliefs and intentions. Some constraining factors were identifaed.