The expectation that teachers will use student achievement data to improve their instruction is a major feature of national and local reform agendas. The theory of action behind data-driven decision making is a mostly causal model of professional action, whereby teachers diagnose weaknesses and implement solutions. The purpose of this article is to examine how high school teachers, situated within their policy and work contexts, use data to inform instructional decisions. Using a framework that draws upon sense-making and co-construction theories on reform implementation, we analyze qualitative data gathered in 4 urban public high schools in the United States. Findings reveal that the process of data use by teachers is complex, multilayered, and influenced by teacher interpretations and social interactions. Teachers used a variety of forms of data to inform their decision making and struggled to reconcile policies promoting data-driven decision making with local beliefs and practices. Implications for research and policy are discussed. (Contains 2 tables and 1 footnote.)