Primary school teachers in England are responsible for the statutory assessment of writing, using a set of criteria. The aim of this research was to analyse the responses of teachers to the criteria "at the expected" standard. An elicitation exercise with 10 primary school teachers was conducted, where teachers talked through their interpretations of the criteria. Qualitative, inductive coding was undertaken to provide a rich and deep understanding of teachers' understandings of the assessment criteria. The perceptions of teachers are shown to be varied and complex. Definitions of the criteria were diverse, and fundamentally different concepts of writing were identified, which points to sources of misunderstanding. Limitations of the assessment criteria themselves are also highlighted, which caused disagreement among the teachers. The findings show evidence of threats to the validity and reliability of the assessment and its use as an accountability measure because of the lack of confidence in inferences drawn from the use of the criteria in the assessment of writing.