The purpose of this study was to investigate whether teachers' engagement in research fosters teachers' professional development and school development. We investigated changes in teachers' professional behavior, the impact of such changes, and factors that affect such changes. Questionnaires were distributed among more than hundred teachers who participated in a course on teacher research in the period from 2013 to 2018. We collected in-depth data by conducting lesson observations and surveying students. Findings showed that teacher-researchers' professional behavior had changed. The changes relate both to professional and school development. Our findings suggest that personal and contextual factors may have contributed to changes in teacher-researchers' professional behavior. Altogether, the findings of this study substantiate the relevance of teacher research for education.