The present study follows a cohort of 17 K-12 teachers through a six-week resident learning experience in science and engineering, and on into the planning and implementation of applications for their classrooms. This Research Experiences for Teachers (RET) program was examined using the strategic approach of design-based research, with its fluid, adaptive management of the complexity of authentic learning "in situ" and its attentive documentation of expected and unexpected events, in process and products, to capture the richness of teachers' and mentors' experiences. Research on effective teacher professional development, adult learning, situated cognition, and learning transfer were utilized to inform the evaluation design. Teachers overall enjoyed the RET experience; they appreciated the faculty expertise and insights, and admired their mentors. In this study the authors see three parts of teacher professional development. The first is knowledge and skills, the intangible tools, cognitive and psychomotor, to do the research tasks. The second is equipment, the physical resources and tangible tools with which to do the work. The third is empowerment--in self-perceptions, motivations, and drive necessary to carry out the plans, provided through initial and ongoing support and communication. Teacher-learners, expert in K-12 science education, learned from university mentors, expert in engineering. Engineer-mentors learned about K-12 science education and the demands of effective teacher development. The result was a change in the practice of both groups of teachers, in K-12 education, in undergraduate engineering (reported by the mentors), and in the adaptive revision of the professional development program itself. (Contains 3 tables.)