This paper reports the main findings of a study that sought to understand how teachers and department heads perceived and experienced the implementation of a classroom observation system in a teacher evaluation context. The data was collected through a teacher and department head survey and interviews with department heads who were responsible for classroom observation. The findings show that respondents regarded classroom observation as an inadequate teacher evaluation instrument. It was also viewed as one of the features of the teacher evaluation system that least contributed to the teachers' professional development. Department heads felt torn between their intermediate management role as teacher observers and their professional role as teacher peers. Cultural norms of individual professional autonomy that regulated relations among colleagues prevented department heads from using information, gathered through classroom observation, to nourish in-depth professional discussions about concrete educational practices.