The impact of scholarly research in education on the educational practice in secondary school is low. Academics examine problems that teachers in school perceive as irrelevant, want to publish in peer-reviewed journals instead of disseminate their work, and aim at generalizing insights rather than improving school practice. Teacher research might be a way to link educational academic research and teaching practice aiming at furthering professionalism in teaching, improving teaching practice and extending the knowledge base on teaching and learning. Four experienced secondary school teachers systematically investigated their classes, guided by a supervisor. Their materials as well as formal and informal communication with their supervisor and with each other were analysed. Related to their research projects, these teachers reported significant changes in their understanding of student learning and their teaching. They took different perspectives on teaching, looked at alternative solutions for problems, and reflected more deeply on their own teaching as well as teaching of their colleagues. Although each of them published an article in a peer-reviewed journal, they reported difficulties with extending the knowledge base on teaching and learning. Not only did they rarely share results with their colleagues in school because they were perceived as external researchers, they also struggled with writing journal articles and coping with peer reviews. We discuss the value of situated generalization as one of the implications and discuss how teacher research could lead to a new in-between research practice, linking theory and practice, and researchers and teachers.