This mixed-methods study examined K-5 teachers' perceptions of using data to inform their instruction, uncovered factors that affect their capacity for understanding data, and described how both of these factors may relate to school improvement. The study outlined factors that supported or impeded teachers' use of data when making instructional decisions. The sample for the study included K-5 self-contained and intervention teachers. Results from the study found the participants regularly use data to inform their instruction, have high levels of confidence with data use, and the local schools of the participants have structures and processes for data review. Participants described supports provided within the school, but also reported challenges and barriers they faced when using data to inform instruction. Lack of time proved to be the most significant barrier related to data use. As the survey and participant interviews revealed there is a significant relationship between the teachers' beliefs about formative assessments and their frequency of use when compared to the lack of use of state and district assessments when informing instruction. With increased accountability, it is imperative educational leaders address teachers' perceptions of using data to inform instruction and understand how teachers prioritize the data they use. Based on the findings, leaders engaging teachers in data-driven decision making and the cycle of data inquiry has the potential to affect school improvement goals and support school organizations with maintaining a culture of data use. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]