Background: Increasingly, teachers and other educators are expected to leverage data in making educational decisions. Effective data use is difficult, if not impossible, without computer data systems. Nonetheless, these systems may be underused or even rejected by teachers. One potential explanation for such troubles may relate to how teachers have made sense of such technologies in practice. Recognizing the interpretive flexibility of computer data systems provides an avenue into exploring these issues. Objective: This study aims to explore the factors affecting teachers' use of computer data systems. Drawing upon the notion of interpretive flexibility, it highlights the influence of sensemaking processes on the use and implementation of computer data systems. Research Design: This comparative case study draws upon interview and observational data gathered in three school districts. Matrices were used to compare understandings about data use and about computer data systems within each district by job role (i.e., central office member, campus administrator, and teacher), as well as across districts. Results: Our findings challenge commonplace assumptions about technologies and their "effects" on teacher work. For example, access to a system or its functions did not determine changes of practice. Paradoxically, we even found that teachers could reject or ignore functions of which they were personally in favor. Although computer data systems can support changes of practice, we found that agency for change rested in people, not in the technologies themselves. Indeed, teachers' sensemaking about "data" and "data use" shaped whether and how systems were used in practice. Although central offices could be important to sensemaking, this role was often underplayed. Conclusion: We provide recommendations regarding how researchers, school, and district leaders might better conceptualize data and data systems. These recommendations include recognizing implementation as an extended period of social adjustment. Further, we emphasize that it is the unique duty of school and district leaders to share their visions regarding data use, as well as to engage in dialogue with their communities about the natures of schooling and data use.