Learning to teach writing is a complex process influenced by many factors. Formative assessment holds promise as a place for preservice teachers to gain a better understanding of students' unique struggles as writers and of writing as a complex, challenging skill. The authors of this article describe how working with a dialogic method of formative assessment gave two preservice teachers unique insights about their students as writers and transformed their understanding of writing development. We argue for the benefits of incorporating more experience with formative writing assessment into the preservice education of English teachers.