The paper reports about a study that examines changes in teachers' incorporation of epistemic practices in their design of engineering lessons and compares them to their views about the nature of engineering knowledge across 100 hours of professional development. Nineteen K-8 teachers in the USA, who were new to teaching engineering as part of a shift in science learning standards, participated. We focused on four areas of engineering practice and epistemology, namely the orientation of engineering toward solutions, the role of context in defining engineering design specifications, the role and nature of data and evidence, and the interdisciplinary nature of science and engineering. Comparisons of changes in teachers' incorporation of epistemic practices show some reflection of changes in their views about the nature of engineering knowledge. Comparisons suggest that they were able to design instruction with appropriate epistemic practices while not necessarily understanding related features of the nature of engineering.