The premise of this article is that excellent teaching requires ongoing assessment of student understanding--a commitment to knowing what students know. The Integrating Mathematics Assessment (IMA) program provides a well-researched example of professional development that helps teachers interpret children's mathematical thinking and guide children toward deeper understanding. The authors describe the features of the program, and provide evidence that it is effective for teachers who wish to use inquiry methods to support students' developing mathematical understandings. The article concludes with a list of recommended resources.