The purpose of this study was to determine how teacher perceptions of data and use of data-driven instruction affect student performance in English language arts (ELA). This study investigated teachers' perceptions of using data in the classroom and what supports they need to do so. The goal of the research was to increase the level of knowledge with up-to-date, specific, and relevant information on effective data-driven instruction. This study was qualitative in nature and grounded in the theoretical framework of action research involving teacher participants in the organization under study. Data collection included two identical standardized Acuity Assessments as well as two sets of teacher interviews. The population of interest was middle school ELA teachers. The sample consisted of scores from middle school students in Grades 6-8 and interviews with their teachers. The data suggested that there is a need for teacher professional development in interpreting and analyzing data and then making decisions regarding how to use the data to inform instruction in the classroom. With this support, teachers were able to use data in the classroom to drive their instruction. There was some change in teacher perception regarding the use of data-driven instruction. Future implications for schools include acknowledging whether data-driven instruction is really happening in the classroom or if it is merely an espoused theory. Implications for students include becoming more data literate, taking ownership of their data, and setting their own learning goals based on that data. Implications for teachers include using data to plan effective lessons and instruction. The data showed that using data is more than useful; it is essential to best practice. [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.]