This study examines how mathematics and science teachers' classroom assessment practices were affected by the National Board Certification process. Using a 3-year, longitudinal, comparison group design, evidence of changes in teachers' classroom practice were measured on six dimensions of formative assessment. The National Board candidates began the study with lower mean scores than the comparison group on all six assessment dimensions; had higher mean scores on all dimensions by the second year, with statistically significant gains on four of the dimensions; and continued to demonstrate substantially higher scores in the third year. Pronounced changes were in the variety of assessments used and the way assessment information was used to support student learning. National Board candidates attributed changes in practice to the National Board standards and assessment tasks. Comparison group teachers who showed noticeable changes in practice described professional development experiences similar to those supported by the National Board Certification process. (Contains 4 tables, 2 figures, and 1 note.)