The research reported on in this paper addresses the problem of preparing Massachusetts middle school mathematics, science, and computer teachers to teach engineering. The results presented here are part of a larger dissertation study investigating six urban public school teachers and their subject matter and pedagogical content knowledge teachers know and develop as they taught an engineering unit. The results discussed in this paper focused on the teachers' instruction and knowledge of the central aspects of the engineering design process (EDP)--the purpose of the EDP, that the EDP is a cyclical, iterative process, and that the EDP fosters communication. The analysis of the videotaped classroom observation data revealed that the teachers as a group covered the central ideas regarding 1. "Purpose of the EDP" and 2. "EDP as a cyclical process" quite well. Every teacher touched upon at least one of the sub-ideas for these two central ideas. The teachers did not cover 3. "EDP for fostering communication" well. Only Frieda and Monica conveyed anything regarding this idea to their students. This paper explores each central idea and the teachers' understandings of them as expressed through their explanations in the classroom observations and follow-up interviews.