The purpose of this paper is initially to present findings which identify components of the practices of teachers acting as assessors of students' mathematics in the normal course of classroom work. At an informal level such practice is found to be complex and intimately related to every aspect of teaching and learning. It is found that even teachers who have undergone some assessment training may underestimate the role of interpretation of evidence, and questions about equity in the uses of teachers' judgements are raised in relation to awareness and practice. It is suggested that more care needs to be taken over the formation and use of `professional judgements' within systems of assessment. [ABSTRACT FROM AUTHOR]