This study provides insight into the quality of practitioner research and the impact of this on the professional development of the individual teacher and the school as a whole. We examined the quality of practitioner research in relation to the goals of the research. We operationalized the quality of the research in terms of the validities outlined by Anderson and Herr. In our study we apply this way of thinking to 11 cases of teacher research in secondary education. We found at least some evidence of outcome validity and catalytic validity for the teachers' practitioner research. However, evidence of process validity and democratic validity was only found in some of the projects. We conclude that the approach adopted in this study can be more generally used to increase the quality and thereby the impact of practitioner research.