In the USA context of the Next Generation Science Standards, elementary teachers face challenges in implementing engineering practices in their teaching due their lack of experience and training in engineering design. The paper reports about a multiple case study that examined one professional development approach to improve teachers' understanding and implementation of the STEM discipline of engineering. Two teams of elementary teachers analyzed their students' written work and assessments during facilitated professional learning community sessions with a science/engineering education researcher after their first implementation of an engineering design unit. The results indicated that the teachers noticed students' understandings and misconceptions about the work of engineers, the disciplinary language for a specific engineering unit, the operational mechanism of a design, and engineering epistemic practices (i.e., envisioning design proposals, making evidence-based decisions, evaluating solutions based on criteria). From this collaborative, focused analysis of student work for the engineering challenge, the teacher teams constructed their understanding of students' thinking and generated recommendations for pedagogical changes in their own STEM teaching practice for engineering design. The findings from this study have implications for forms of professional development that sustain teacher learning about the STEM discipline of engineering design.