Assessment, particularly formative assessment, is critical to knowing how a student is performing academically and how best to support that student. Teachers, especially new teachers, need ongoing support in developing and using both summative and formative assessments as an integral part of their instructional practice. Written collaboratively by a researcher, school coach, and teacher, this article offers a framework for developing and improving assessments. It examines how a small urban high school focused on improving assessments across the curriculum. It describes how the teacher, working with the school coach, developed his assessment practice, especially of students' writing, and how work with teachers is connected to a schoolwide staff development program. Consistent with the authors' framework for developing classroom assessments, the staff development program makes teachers' assessments public in order to share promising practices and support reflection. The article argues for a comprehensive, ongoing, multilevel approach to developing classroom assessment practice. (Contains 3 figures.)