Interdisciplinary STEM programs are in demand for United States middle schools (ages 11 to 13 years) and high schools (ages 14 to 18). The Real STEM Project collaborated with 12 schools to develop and implement such programs. We open with a description of the project, including the 21st century STEM reasoning abilities that were proposed as learning outcomes for the STEM programs. We then focus on one of the five reasoning abilities, engineering design-based reasoning, since engineering often serves as a driver for STEM programs. An exemplar of engineering design as a driver for STEM from one of the participating teacher’s classrooms is provided, and a summary of teaching practices supporting interdisciplinary STEM is drawn from the example.