Standardized testing for the purpose of accountability continues to dominate our nation’s schools. Since we first reported on the Mathematics Assessment Collaborative [Foster and Noyce 2004], states have responded to the stringent testing requirements of the No Child Left Behind legislation by expanding annual testing in reading and mathematics to all students in grades 3 through 8. Currently states are adding science tests and more tests in high school. Working under cost constraints, most states elect to use multiple-choice tests, while some commentators such as Peterson [2006] detect a “race to the bottom”—a tendency to lower standards and simplify tests as a way of ensuring that more and more students can be deemed proficient. (See also [Fuller et al. 2006].)

How has assessment been used to inform instruction? A number of districts, challenged urban districts in particular, have responded to the need to boost student scores by increasing the frequency of benchmark assessments. Some districts developed assessments aligned with local curricula to help ensure that coverage and learning across schools. Other districts invested in technology-based programs that offer quarterly updates on student progress along a linear scale, based on easily scored (but often skills-oriented) computer multiple-choice assessments. These programs, while they may reassure a school’s staff about student progress or alert them to trouble ahead, do little to inform teachers about how students are thinking, what they understand, where they are falling down, and how, specifically, teachers might change their own instructional practices to address students’ difficulties.

For the past nine years a group of school districts in California’s Silicon Valley have taken a different approach to mathematics assessment. These districts have supplemented the state testing system with a coordinated program of support and learning for teachers based on a common set of assessments given to students. In this chapter, we briefly review the history of the Mathematics Assessment Collaborative (MAC). We describe how the results of the annual performance assessment are used to guide professional development. We offer additional examples of what the MAC is learning about student understanding across the grades. We review trends in student performance and discuss the relationship between student performance on the MAC assessment and on the state tests.