In the past decade, there has been a growing interest in scientific practices as a reform focus in K-12 science education of the United States. In this context, scientific practices refer to practices that have family resemblance to scientists' professional practices and simultaneously are pedagogically accessible and useful to students. In this study, I propose development of students' epistemic agency as an overarching goal for this practice-based approach to science learning. In particular, I argue that students' epistemologies, one dimension of epistemic agency, should be developed as a result of participating in practice-based science learning. The research within this dissertation focuses on studying the practice of scientific modeling. There is a body of prior studies on students' epistemological understandings about models and modeling. None have examined how students' epistemologies about modeling changes over time and why they change the way they do. This research aims to contribute to this body of work by investigating how three elementary students' epistemologies as deployed in their modeling practice, or, their "epistemologies in modeling" (EIMs) changed over time as a class of 5th-grade students (N=24) and their teacher, Mrs. M, and an intern teacher, Ms. H, enacted a model-based curriculum unit about evaporation and condensation and ways in which some of the curriculum events influenced the changes of their EIMs. To achieve these goals, I conducted a microgenetic analysis of the three focus students' EIMs from the models, utterances, and notes they made in nine modeling activities as part of their curriculum enactment, and analyzed ideas about modeling from some modeling-related curriculum events that preceded each modeling activity using a coding scheme I developed based on prior analytical frameworks and the data. Analysis indicates that the students attended to three main model features such as communicative features (e.g., labels, sentences, key, colors), microscopic/theoretical entities (e.g., water particles), and empirical data (e.g., percentage humidity) with varying epistemic ideas about modeling throughout the unit. The students began with nascent epistemic ideas that emphasize clarity and including many details, but as they gained more experience with modeling, they developed more advanced epistemic ideas related to providing a scientific explanation (mechanism) and making a model accurate and persuasive. The curriculum materials, teachers' instructions and scaffolding, and students' interactions played important roles in the development of the focus students' EIMs. These findings provide some insights into elementary students' epistemologies about modeling that can contribute to learning progression research for scientific modeling. First, this study suggests that we need to attend to epistemic ideas that elementary students have in common as a result of sharing a fairly homogeneous historically established sociocultural world in developing a learning progression for modeling. Second, by showing an intermediary state that elementary students had as they developed their epistemologies about modeling, this study provides an insight into a trajectory or mechanism of how students' epistemologies about modeling become increasingly sophisticated. I hope that this work contributes to the large effort to help students become more active and capable epistemic agents by learning science from engaging in scientific practices both for their present science learning and for their future life they will live as citizens in societies that will be increasingly populated with complicated, controversial socioscientific issues. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]