Learning assistants (LAs) are peer educators who work alongside faculty to facilitate active learning activities and help students develop conceptual understanding. LAs and LA training programs are becoming increasingly prevalent in science, technology, engineering, and mathematics (STEM) gateway courses in the United States, and research continues to document the positive impacts on students and LAs in LA-facilitated classrooms. While evidence accumulates regarding positive student impact, relatively less work has investigated the ways LAs themselves might also be impacted by their participation in these programs. To improve LA training and student learning in STEM, it is vital that educators better understand the ways the experiences of LAs can promote personal and professional growth as members of the STEM community. To address this gap, we explored the epistemological development of LAs who participated in an LA program (i.e., pedagogy training and classroom practice) at a large, public, teaching-focused university. LAs participated in semi-structured interviews at the beginning and end of the academic term and completed bi-weekly reflections where they described impactful events and challenges. Using qualitative coding, we operationalized Baxter Magolda's epistemological reflection model to identify the impactful experiences that provoke epistemological development or reorientation. Our findings highlight three impactful experiences that help promote epistemological development for LAs: (i) recognizing the importance of language and listening; (ii) observing more knowledgeable others struggle with content; and (iii) providing students with the "wrong" answer to a question. We argue that these experiences can promote epistemological development in ways that help LAs become more effective in facilitating student learning and, ultimately, more thoughtful members of STEM disciplines. By working to create spaces for LAs to have these impactful experiences in training and preparation, educators can promote epistemological growth in ways that benefit both LAs and the students they serve.