Due to educational policy, data use in schools is a widespread method of school improvement, yet results remain inconsistent. Across the literature, the school leader is a key determinant of the effectiveness of data use. This study sought to shed light on the role of the school leader in data use through a mixed methods case study. The study included two PreK-2nd grade schools from a school district in Long Island, New York. I administered a survey on data use attitudes and practices to both principals and teachers. I also conducted interviews with both principals and two teachers from each school. And I observed four Professional Learning Communities (PLCs) meetings. Analysis of survey results was conducted through a t-test and ANOVA and coding was used to analyze the transcripts from interviews and observations. Results revealed that both teachers and principals reported feeling high levels of support for data use. Teachers perceived formative data the most useful to their practice, rating it higher than interim or summative data. Collaborative teacher time was used for data use, but the method employed varied from the traditional inquiry cycle described in the literature. Based on these findings, I propose a new model of data use, the Do, Know, Now model, which reflects observed data use both in this study and another. In the Do, Know, Now model, teachers (DO) use existing data to identify a problem, (Know) leverage teacher expertise to consider solutions, and (Now) select a solution for immediate implementation. The Do, Know, Now model accommodates the constraints of everyday life in schools and may be explained by a satisficing decision-making model wherein the decider stops searching once a good enough solution has been found. The development of a data use model reflective of actual practice is significant for school leaders who seek to use data use to improve student outcomes. By understanding the model teachers are using, school leaders can provide better support. For example, principals may invest in high-quality ready-to-use materials for teachers to select from. [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.]