Reform efforts are often unsuccessful because they failed to understand that teachers play a key role in making educational reforms successful. This paper describes a long-term teacher professional development (PD) program aimed at educating and training teachers to teach interdisciplinary topics using case-based method in science. The research objective was to identify, follow and document the processes that science teachers went through as they assimilated the interdisciplinary, case-based science teaching approach. The research accompanied the PD program throughout its 3-year period. About 50 teachers, who took part in the PD program, were exposed to an interdisciplinary case-based teaching method. The research instruments included teacher portfolios, which contained projects and reflection questionnaires, classroom observations, teacher interviews, and student feedback questionnaires. The portfolios contained the projects that the teachers had carried out during the PD program, which included case studies and accompanying student activities. We found that the teachers gradually moved from exposure to new teaching methods and subject matter, through active learning and preparing case-based team projects, to interdisciplinary, active classroom teaching using the case studies they developed.