In the context of significant reforms to the curriculum for New Zealand schools, this article presents an early line of findings from an ongoing ethnographic multi-case study investigating New Zealand primary school teachers' enactment of informal formative assessment. Data were gathered through observations, interviews, and document collection in one Auckland primary school classroom. The findings illustrate the value of informal formative assessment and provide a perspective on the use of formal assessment tools grounded in classroom practice. They suggest some barriers to greater student involvement in assessment, and identify trust, perceptions of validity, control, and definitions of quality as key considerations in how teachers enact formal assessments.