This paper concerns teacher learning. It describes the impact on teacher knowledge and reported practice of working with diagnostic assessment tools for writing. These tools were specifically designed with a dual purpose: to provide diagnostic information about students to guide teaching for enhanced learning outcomes and also to develop teacher understanding about features of, and developmental progressions in, writing. We describe the features of the tools which facilitate teacher learning. Then we present three small-scale studies conducted during and after development of the tools to explore their impact. Discussion focuses on teachers' reports of their learning about writing and how working with the assessment tools impacted a range of teaching practices. An Example of a writing assessment rubric is appended. (Contains 1 table.)