We wanted to assess the quality of research conducted by Dutch secondary education teachers. We therefore needed to investigate the criteria for evaluating teacher research. The research timeline of this study covered four school years. We analyzed research reports and evaluation forms. We found that five quality criteria are useful for assessing teacher research. We made concrete operationalizations of these five criteria and their indicators. We found that successfully conducted teacher research contributes to professional and school development. Its contribution to professional development is greater and more concrete than to school development.