There is a need to change the teaching methods of the science subjects. International surveys, e.g. TIMMS and PISA, have been showing relatively declining skills for the Swedish students in the science subjects. International science education research has found good examples of teaching and learning, but the research stays within the research communities and does not reach the teachers and their teaching. The gap between research results and teachers' practices in the classroom is the basis of this investigation. Research shows that the teacher is one of the most important factors for student learning, therefore, this study has put great emphasis on the teachers' competencies. The focus of this investigation is the relations between teachers' knowledge of the Nature of Science (NOS), the Nature of Science Inquiry (NOSI) and inquiry-based teaching of Science.The project follows longitudinally two teachers as they take part in a researchbased implementation process of predesigned inquiry-teaching sequences in Physics. The context is a secondary school in Sweden (grades 8-9, age 14-16 years). The project is set around group discussions between the involved teachers and the researcher on planning, implementing and analysing actual inquiry teaching. The results describe possibilities and obstacles concerning the implementation of inquiry teaching as perceived by the teachers. Having navigated obstacles the teachers saw great potential in the inquiry model based on students' motivation and learning. The results of the project provide indications on how future in-service teacher courses in Science could be designed.vi