In this article, the authors describe a collaborative effort involving 30 school districts in California's Silicon Valley that are seeking to overcome the ill effects of mandatory high-stakes standardized testing in mathematics. These districts administer, score, and analyze a common set of performance assessments in mathematics in a way that guides professional development and leads to changes in teaching strategies. The assessment tasks place particular value on student understanding and knowledge transfer to new problems. Growth in students' test scores on statewide standardized tests occurs almost as an incidental effect of this approach. The authors report that the districts participating in the Mathematics Assessment Collaborative have seen improved mathematical understanding on the part of both their students and their teachers. (Contains 2 figures and 8 endnotes.)