Science-Technology-Environment-Society (STES) orientation in science education is currently being implemented in Israeli high schools within the framework of 'science for all' reform worldwide. This paper focuses on assessing the conceptual change of teachers who have been involved in the development, implementation, field-testing and evaluation of several modules. These modules constitute a grade 10-11 high school national curriculum titled STEMS - 'Science, Technology, Environment in Modern Society'. STEMS is aimed at developing an autonomous learner, capable of system thinking, decision making and problem solving within the real life STES context. We sensed that the intrinsic nature of STEMS curriculum requires that the teachers, who will teach it, will also be the developers of its modules. Involvement of this kind makes the teachers responsible for their own conceptual change, explanations and interpretations. Our formative evaluation indicates that the conceptual change of STEMS teachers was gradual. Participants differed with respect to what sort of 'treatment' or experience within the project actually affected who and when. It was apparent that the change occurred with respect to both their content knowledge and pedagogical views. A positive response towards teaching beyond the discipline boundaries was followed by teachers' active involvement and participation in the development process and team discussions. Thus, the STEMS project affected their teaching/learning perception towards interdisciplinarity. These findings are in accord with teachers' support of a life cycle approach for curriculum development as being suitable for achieving the STEMS objectives. The teachers emphasized the need to practice together with their students scientific inquiry and experiment design skills which, foster an autonomous learner. At the end of the first year of the curriculum development process, STEMS was finally conceptualized by the project teachers as a novel way of learning, rather than another sophisticated teaching technique. The major conceptual change was, the switch teachers made from the role of knowledge providers into that of learners. The interplay among action, participation and conceptualization turned out to be instrumental in our life cycle approach for developing the STEMS curriculum.