The purpose of this case study was to examine one school district's 8th grade mathematics teachers' perceptions, analysis of, and use of their own students' formative interim data. Research has shown that teachers have difficulty with data literacy, and are not getting the trainings they need. Unfortunately, this may arise from the fact that a majority of teacher training programs and school systems generally do not fully address data skills and data informed decision making processes (Means et al., 2010). Using a "Data Literacy for Teaching" conceptual framework constructed by Gummer and Mandinach (2015), I designed and conducted a multi-case study in a small school district to better understand all of the 8th grade mathematics teachers' abilities and understandings around data literacy. I used Gummer and Mandinach's conceptual framework and the included "Inquiry Cycle" to analyze data around the teachers' implementation of one interim assessment in Fall 2019. The sources of data in this study were a questionnaire given at the beginning of the school year, a data conference interview after the administration of an state provided interim assessment (where the student formative data was collected), and an observation (with two short interviews surrounding it) a few days after the assessment to capture the teachers' actions after their analysis of the student data. The study yielded results that teachers may use two different components of data literacy: analyzing data, and using data. These two components of data literacy appear to be distinct and although both are needed to maximize the use of formative assessments, a knowledge of one does not mean there is knowledge of the other. In my analysis, I created a "Characterization Framework" to help understand where teachers were in their data literacy journey and where more opportunities lie. The results show that out of six teachers, three of these teachers were still "developing" in both "analyzing data" and "using data." Two teachers were "developing" in "analyzing data," but were labeled "efficient" in "using data," and one teacher was "efficient" in "analyzing data," but was "developing" in "using data." Previous research on perceptions of data and how it impacts data use was confirmed in that teachers that did not value the use of data, were some of those that were labeled "developing" in the use of data. Additionally, even within this small district, the 8th grade teachers varied widely on use, analyzation and perception of data. This research contributes a qualitative view of how middle school mathematics teachers understand and use data. Results fill a gap in the literature about how the Data Literacy for Teaching framework can be useful to understand teachers' data literacy. Implications for professional development and how teachers can grow in data literacy are discussed. [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.]