In recent years educators and educational researchers in the U.S. have begun to introduce engineering design activities in secondary science classrooms for the purpose of scaffolding science learning as well as supporting such general problem-solving skills as decision making and working in teams. However, such curricula risk perpetuating a misconception about the nature of engineering design, or technology more broadly, as being applied science, when in fact its nature is more complex and its relationship with science is reciprocal. Developing an understanding of technology itself is a major part of national science education standards. Based in situative and cognitive theories, this paper critiques assumptions of engineering design activities in science classrooms from the perspective of their alignment with an epistemology of professional engineering design. Analyzing alignment of professional and classroom practices will require thick descriptions of both realms. This paper contributes to such an analysis, drawing on philosophical and sociological accounts of engineering practice to develop a prototypical epistemology of engineering design to which classroom practice may be compared, or that may be considered in developing future classroom engineering design activities. (Contains 1 footnote.)