As engineering learning experiences increasingly begin in elementary school, elementary teacher preparation programs are an important site for the study of teacher development in engineering education. In this article, we argue that the stances that novice teachers adopt toward engineering learning and knowledge are consequential for the opportunities they create for students. We present a comparative case study examining the epistemological framing dynamics of two novice urban teachers, Ana and Ben, as they learned and taught engineering design during a four-week institute for new elementary teachers. Although the two teachers had very similar teacher preparation backgrounds, they interpreted the purposes of engineering design learning and teaching in meaningfully different ways. During her own engineering sessions, Ana took up the goal not only of meeting the needs of the client but also of making scientific sense of artifacts that might meet those needs. When facilitating students' engineering, she prioritized their building knowledge collaboratively about how things work. By contrast, when Ben worked on his own engineering, he took up the goal of delivering a product. When teaching engineering to students, he offered them constrained prototyping tasks to serve as hands-on contexts for reviewing scientific explanations. These findings call for teacher educators to support teachers' framing of engineering design as a knowledge building enterprise through explicit conversations about epistemology, apprenticeship in sense-making strategies, and tasks intentionally designed to encourage reasoning about design artifacts.