Engineering design-based STEM integration is one potential model to help students integrate content and practices from all of the STEM disciplines. In this study, we explored the intersection of two aspects of pre-college STEM education: the integration of the STEM disciplines, and the NGSS practice of engaging in argument from evidence within engineering. Specifically, our research question was: While generating and justifying solutions to engineering design problems in engineering design-based STEM integration units, what STEM content do elementary and middle school students discuss? We used naturalistic inquiry to analyze student team audio recordings from seven curricular units in order to identify the variety of STEM content present as students justified their design ideas and decisions (i.e., used evidence-based reasoning). Within the four disciplines, fifteen STEM content categories emerged. Particularly interesting were the science and mathematics categories. All seven student teams used unit-based science, and five used unit-based mathematics, to support their design ideas. Five teams also applied science and/or mathematics content that was outside the scope of the units' learning objectives. Our results demonstrate that students integrated content from all four STEM disciplines when justifying engineering design ideas and solutions, thus supporting engineering design-based STEM integration as a curricular model.