Contribution: This paper reports engineering students' practical epistemic cognition by studying their interactional work in situ. Studying “epistemologies in action” the study breaks away from mainstream approaches that describe this in terms of beliefs or of stage theories. Background: In epistemology, knowledge is traditionally seen as “justified true belief,” neglecting knowledge related to action. Interest has increased in studying the epistemologies people use in situated action, and their development of epistemic fluency. How appropriate such approaches are in engineering and design education need further investigation. Research Questions: 1) How do students in the context of a design project use epistemic tools in their interactional work? and 2) What are the implications of the findings in terms of how students' cognitive and epistemological development could be conceptualized? Methodology: A collaborative group of six students were video recorded on the 14th day of a fifth-semester design project, as they were preparing for a formal critique session. The entire, almost 6 h, session was recorded by four video cameras mounted in the design studio, with an additional fifth body-mounted camera. The video data collected was analyzed using video ethnographic, conversation analysis, and embodied interaction analysis methods. Findings: The results show that the students use a wealth of bodily material resources as an integral and seamless part of their interactions as epistemic tools, in their joint production of understanding and imagining. The analysis also suggests that students' epistemological and cognitive development, individually and as a group, should be understood in terms of developing “epistemic fluency.”