Background: A growing number of studies argue that data use practices in schools have not sufficiently attended to teachers' learning about students, subject matter, and instruction. The result has been changes in instructional management (e.g., student grouping, assignment of students to tutoring) rather than instructional improvement. Further, there is a paucity of research on how teachers make sense of data and their ensuing instructional actions. Purpose: We report findings from qualitative research on an intervention designed to put teacher learning about mathematics instruction center stage in data use practices. The research sought to understand what happened as teachers made sense of data in their professional learning communities (PLCs), what changes they made in their mathematics instruction, and why they made the changes. Research Design: The theoretical foundation for the research is situative theory, which conceptualizes teacher growth as "a process of increasing participation in the practice of teaching, and through this participation a process of becoming knowledgeable in and about teaching." A case study approach was chosen to illuminate the complex interrelationships among intervention components and their influence on teachers: (1) between individual teacher sensemaking about data and collective sensemaking in PLCs and (2) between sensemaking and instructional changes. Additionally, case study methodology facilitates theory building grounded directly in data by providing nuanced accounts of the phenomena under study that uncover concepts and coherently relate them to one another. Teacher interpretation of data is ripe for theory building. Findings: The case study of Ms. Walker illustrates in rich detail the developmental nature of her growth and the important roles of dissonance, collegial discussion, and productive dissonance in that process. Due to considerable progress in both her questioning strategies and her ability to build on student thinking to focus on important mathematical ideas, Ms. Walker was able to move beyond surface instructional adjustments to demonstrate substantial instructional improvement. Conclusion/Recommendations: We argue that a fuller understanding of how teachers experience dissonance, and the supports necessary to make that dissonance productive, can enrich the design and implementation of data use practices. The research also offers an example of the contribution that microprocess studies can make to research on data use practices. We encourage researchers to attend carefully to teacher sensemaking and interrogate the concepts of dissonance and productive dissonance in future theory building about data use practices.