This article reports on a mixed methods study into the development of research knowledge of secondary education teachers conducting research in the context of a professional development program. 26 teachers of 12 schools in the Netherlands participated in the study. Data were collected by using questionnaires, interviews, concept maps, oral tests and logbooks. Findings show a positive research knowledge development in teacher-researchers after following a one-year course in teacher research. This development was not only found in teachers’ self-reports, but was also measured by tests. In the process of research knowledge development, teacher-researchers mainly have difficulties with formulating research questions, developing research instruments, and reporting about their research.