Many secondary math and science teachers don't understand the nature and application of engineering adequately to transfer that understanding to their students. Research is needed that investigates and illuminates the process and characteristics of development that addresses this gap. This mixed-method study examines the developmental experiences of 16 public math and science teachers in a yearlong engineering experience. Teachers partnered with university engineering professor-mentors for six weeks on campus followed by ongoing support for transfer to their secondary classes (grades 9-12). Teachers gained engineering knowledge and skills, and an understanding of engineering's diversity and interdisciplinary nature, plus integrative applied strategies for math and science instruction. They developed productive perceptions (of value, utility, benefits, feasibility and fit) of engineering for their grade 9-12 students, along with efficacy and intent to use what they had learned. Teachers identified positive, profound shifts in their perceptions of engineering and engineers. All teachers integrated engineering principles into their classrooms, but the process was more challenging for math teachers than for science teachers. This developmental experience may help bridge gaps of school-to-life and disciplinary boundaries that separate formal educational environments from authentic life experience.