"Data use" and "data-based decision making" are increasingly popular mantras in public policy discourses and texts. Policy makers place tremendous faith in the power of data to transform practice, but the fate of policy makers' efforts will depend in great measure on the very practice they hope to move. In most conversations about data use, however, relations between data and practice have been underconceptualized. In this essay, I identify and discuss some conceptual and analytical tools for studying data in practice by drawing on work from various theoretical traditions. I explore some ways in which we might frame a research agenda in order to investigate data in everyday practice in schools. My account is centered on schoolhouse work practice, but the research apparatus I consider can be applied to practice in other organizations in the education sector and indeed to interorganizational practice, a critical consideration in the education sector. (Contains 1 note and 4 figures.)