The idea of data use as an educational reform strategy has gained significant traction in recent years, but there is limited research that illuminates what actually happens when teachers use data in practice. This study investigates the ways a group of middle school mathematics teachers used data in their classrooms and as part of an action research study group. Employing the theoretical framework for data use developed by Coburn and Turner (2011), this study takes into account multiple, often overlapping, influences on teacher data use, including teacher evaluation policies and teachers' engagement in action research. Drawing on interview and observation data across a full school year, this study seeks to answer questions about "what" data teachers identified as valid evidence, "how" they discussed data and evidence, and what state, district, and school "policies" and "initiatives" might have shaped their understandings of data. Additionally, this study adopts an ecological perspective of agency (Biesta & Tedder, 2007) to investigate how teachers did and did not express agency as a part of their discussions of data use. Findings indicate that the ways teachers used and discussed data were closely tied to the audiences with whom they interacted. The final chapter of this study summarizes the complex ways teachers use data in practice and how these findings relate to prior research. Recommendations are made to clarify perceived meanings of "data" and "data-driven" and the role of action research as an intervention to promote data use is considered. Theoretical contributions of this study are also discussed along with implications for future research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]