Abstract This article presents the findings of an exploratory study by the Netherlands’ Inspectorate of Education, which investigated whether unannounced school inspections can strengthen the quality and impact of educational supervision. Although inspections in the Netherlands are traditionally announced in advance, this study examines the potential differences in judgements regarding educational quality between unannounced and announced visits, as well as the associated advantages and disadvantages. The research methodology comprises a systematic review and a pilot study conducted at primary and secondary schools. Following PRISMA principles, the systematic review identified thirteen relevant studies indicating that the impact of unannounced inspections varies by supervision type. The pilot study involved unannounced and announced visits to the same schools to enable direct comparison of the assessments. This was supplemented by interviews with school leaders, teachers, board members, coordinators, and inspectors. The findings demonstrate no reason to assume that unannounced school inspections lead to different assessments to those of announced inspections. Furthermore, the results suggest that the advantages and disadvantages of unannounced inspections are multifaceted, with certain advantages also presenting as disadvantages. To conclude, in education unannounced inspections are not necessarily more effective than announced inspections. However, this article identifies the ways in which unannounced visits can be beneficial and the cases in which they are useful. Supervisors must consider what is important to them.