By providing sustained, embedded professional development over the course of a school year--as well as involving teachers in creating high-quality assessment tasks, implementing the assessments with students, and tying assessment directly to instruction--the authors increased the value that their teaching community places on mathematical reasoning. They also supported teachers in living out that value in the classroom. Students emulate what their teachers model. In this article, the authors discuss a vehicle for instructional transformation and describe how they created and implemented assessment tools that support student reasoning. (Contains 7 figures.)