The effects of school inspections on school improvement have been investigated only to a limited degree. The investigation reported on in this article is meant to expand our knowledge base regarding the impact of school inspections on school improvement. The theoretical framework for this research is partly based on the policy theory behind the Dutch Educational School Supervision Act (the latter includes assumptions about how school inspections lead to school improvement). Interviews and a survey with school inspectors gave insight into how school inspectors implement the Supervision Act and how they assess schools, and stimulate schools to improve. The results of ten case studies showed that all schools started to improve after a school visit. The innovation capacity of the school and the school environment do not seem to contribute to school improvement after school inspections. No effects were found on school-improvement processes of the number of insufficient scores that schools received from inspectors, the extent of feedback and suggestions for improvement, and the number of agreements. The provision of feedback about weaknesses, the assessment of these weak points as unsatisfactory, and the agreements between an inspector and the school regarding improvement activities do appear to make a difference in promoting school improvement.