The purpose of this exploratory study was to examine the use of interim assessments and the policy supports that promote their use to change instruction, focusing on elementary school mathematics. The authors use the term "interim assessments" to refer to assessments that: a) evaluate student knowledge and skills, typically within a limited time frame; and b) the results of which can be easily aggregated and analyzed across classrooms, schools, or even districts (Perie, Marion, & Gong, 2009). Their study looked at how 45 elementary school teachers in a purposive sample of 9 schools in 2 districts used interim assessments in mathematics in 2006-07. The study focused on teachers' use of data in a cycle of instructional improvement; that is, how teachers gather or access evidence about student learning; analyze and interpret that evidence; use evidence to plan instruction; and carry out improved instruction. It also considered the many factors that influence how teachers access, manage, interpret, and act on data. These include district and school policies and practices, and organizational norms and routines, as well as educator capacity. The two study districts--Philadelphia and Cumberland, Pennsylvania--share a common accountability context (i.e., the same state standards and state test), use the same elementary mathematics program, "Everyday Mathematics" ("EM"), and had adopted interim assessments in elementary mathematics. By selecting one urban and one suburban district, they sought to learn how policy supports for assessment and instructional improvement function in these different environments. Within each district, they chose schools that had made Adequate Yearly Progress but reflected a range of student performance as well as the ethnic and socio-economic diversity of the district. Seven of the nine study schools were Title I schools. In each site, they conducted interviews with district administrators, principals and instructional support staff. In each school, they observed and interviewed all 3rd- and 5th-grade teachers, their focal grades. The authors collected information on how the teachers analyzed and acted on their interim assessment data and how they would respond to student errors on assessment items. They also conducted a short survey designed to measure participating teachers' mathematical knowledge for teaching (Hill, Shilling, & Ball, 2004). This policy brief reviews the study's key findings regarding the policy supports that existed to support data use and teachers' actual use of interim (and related formative) assessment data. It also presents implications for educators, policymakers, and researchers. The authors' findings highlight the potential and limitations of interim assessments for the four stages of the instructional improvement cycle. They conclude that interim assessments that are designed for instructional purposes are helpful but not sufficient to inform instructional change. When well-supported by their districts and schools, teachers used interim assessment data to decide what to re-teach to and to whom, but not necessarily to change the ways in which they taught this content. Rather, teachers' instructional and assessment practices appeared to be affected more by their capacity to understand their students' mathematical learning than by the type of assessment (interim or formative) they used. (Contains 2 footnotes.)