One of the fundamental practices identified in Next Generation Science Standards (NGSS) is argumentation, which has been researched in P-12 science education for the previous two decades but has yet to be studied within the context of P-12 engineering education. This research explores how elementary and middle school science teachers incorporated argumentation into engineering design-based STEM (science, technology, engineering, and mathematics) integration curricular units they developed during a professional development program. To gain a better understanding of how teachers included argumentation in their curricula, a multiple case study approach was conducted using four STEM integration units. While evidence of argumentation was found in each curriculum, the degree to which it appeared in each case varied. The strongest potential for argumentation occurred when students were required to explain and justify their final engineering design solutions to the client; certain guiding questions and discussions also promoted argumentation, depending on their structure. Additionally, argumentation was found to support engineering concepts such as the process of design, engineering thinking, communication in engineering contexts, and the application of science, mathematics, and engineering content. These findings support the idea that argumentation can be integrated into P-12 engineering education contexts in order to support students' STEM learning. [This paper was previously published in the Proceedings of the 2015 American Society for Engineering Education Annual Conference & Exposition.]