Despite data-driven decision making being a ubiquitous part of policy and school reform efforts, little is known about how teachers use data for instructional decision making. Drawing on data from a qualitative case study of four elementary schools, we examine the logic and patterns of teacher decision making about differentiation and ability grouping. We find that district and school policies conditioned teachers' decision making through mandated time for instructional differentiation, curricular tools, and online program adoption. Educators used various strategies reflecting different logics, types of data used, and sources of decision making. Implications for theory and research are discussed.