In order to enhance teachers' professional awareness, it is necessary to understand and value their subjective or personal knowledge and beliefs. This study investigated the change of science teachers' personal knowledge about teaching models and modeling in science within the context of educational reform in The Netherlands. The study followed nine experienced science teachers during the first years of the implementation of a new syllabus, which emphasizes models and modeling. Data collection consisted of the repeated administration of a Repertory Grid instrument. From the results, three different types of personal knowledge concerning teaching models and modeling in science were identified, each of which showed significant change over time. Type 1 combined modeling as an activity undertaken by students with the learning of specific model content. In Type 2, the learning of model content was combined with critical reflection on the role and nature of models in science. Finally, in Type 3, the learning of model content involved both students' production and revision of models, and a critical examination of the nature of scientific models in general. Implications for the teachers' professional development are discussed. (Contains 4 tables and 4 figures. Appended to this document is "The Rep Grid Procedure.") [Funding for this project and article was provided by Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO, The Netherlands).]