Teaching in secondary education is mostly grounded in the practical wisdom of teachers. In general, teachers have limited knowledge of, access to, and interest in insights from scholarly work. Teacher research might be a way to move beyond practical wisdom as the only basis for good teaching. This study aimed to explore whether teacher research can encourage teachers' learning process as professionals, improve teaching practices, and generate knowledge about these practices. Participants were 44 experienced secondary school teachers in the Netherlands following a 2-year Master of Science professional development program on teaching and learning. Data was gathered using questionnaires, learner reports, a group interview, and participants' master theses. Thematic analyses of the combined data sources showed that the teachers report significant changes in the way they teach and think about teaching: they mentioned that they learned to take different perspectives when they are confronted with problems in their teaching and they reported being more focused in their teaching practice on what they want to change in their teaching. Concerning the third aim of generating knowledge, they mentioned challenges that are common for starting researchers such as how to formulate a researchable question, how to select relevant literature, how to deal with peer review, and how to perform situated generalization. We discuss what kind of educational research is valuable for teachers as learning professionals in secondary schools.