The purpose of this study was to understand the use of interim assessment data to inform mathematics instruction. A qualitative case study methodology was used to analyze the use of interim assessment data in a suburban elementary school. Data sources included interviews with three teachers at varying levels of their profession and the principal; observations of teachers working with assessment data in teams or as part of a professional development setting; artifacts representing interim assessments, supporting materials, and analysis results. The study explored the type of culture that was developed around data use, the types of assessments that were used, how the data were analyzed, and teachers' use of the data as a means to inform their instruction. The results from this research will inform the work of teachers and administrators as they design systems to support students' learning.