The principle-based approach is at the heart of the inspection philosophy of the Netherlands Inspectorate of Education. In this paper, we present the results of complementary qualitative and quantitative studies analysing the impact of this approach. In the first study, we apply a system dynamics approach to provide insight on the impact of the principle-based approach in schools. The second study builds on these findings and uses quantitative survey data to "test" the various ways and mechanisms by which the respondents perceived principle-based approach affects schools. Together, these complementary studies provide a deeper understanding of the impact of the principle-based inspection approach on educational quality and beyond. Overall, we find positive effects of the principle-based approach. The qualitative study reveals variation in mechanisms within schools that affect educational quality. These findings appear to be robust in our quantitative analyses; the effect sizes are small and curvilinear.