Early childhood research and policy have promoted the use of language and literacy assessment data to inform instruction. Yet, there is a limited understanding of preschool teachers' data practices and sensemaking, particularly when considered from the perspectives of practicing teachers. In this multicase study, we used a phenomenological approach to generate a theory about preschool teachers' data practices in relation to supporting children's language and literacy outcomes. Twenty preschool teachers participated in a series of three observations, planning interviews, and stimulated recall interviews designed to tap their pedagogical reasoning and data use practices. The framework that emerged through iterative within- and cross-case analyses comprised three major elements (what teachers knew, how they knew it, and the way they used the data) and suggested that teachers could be characterized into three data use profiles (data gatherers, in-the-moment data users, and integrated data users). Findings indicate (a) teachers may understand data differently than researchers or policymakers do, (b) teachers' understanding of data sources goes beyond traditional conceptualizations, (c) a continuum in teachers' data use practices, and (d) a need to better support teachers in moving from simply doing assessment to using data in ways that are meaningful for practice and children's language and literacy outcomes.