This paper discusses engineering praxis ethos (EPE), a proposed framework for constructing a STEM learning environment embedding interrelated components of learner experience and design activity, which can support curriculum and instructional design and evaluation in STEM education. The authors propose that STEM is a meta-discipline that relies on the design life cycle, the intellectual root of engineering disciplinary knowledge. The nature of design and design activity calls attention to domain-specific needs of STEM teachers and focuses discussions and critiques about the epistemological adequacy of integrated STEM content knowledge. Given the urgent need for learners to develop 21st century skills in STEM, the authors argue for the feasibility of incorporating both mathematical and simulation models using new technologies when designing experiences for learners. An example of teaching and learning systems thinking in undergraduate engineering education highlights how EPE can be a viable theory of action for STEM educators.