This chapter presents the institutional sclerosis thesis, which specifies the relation between polder politics and institutional reform. It evaluates the hypothesis and suggests an alternative interpretation of the relation between polder politics and institutional reform: polder politics is not so much a barrier to institutional reform, but a prerequisite for it. The institutional-sclerosis thesis explains the lack of institutional reform under polder-political conditions. The explanation for successful reforms under polder-political conditions relates to the content of the proposals. In the case of the reorganisation of the Dutch police force, policy entrepreneurs also tried to utilise a 'political window'. The chapter also presents some case studies to illustrate the limitations of using a top-down policy style for institutional reforms in governmental settings that are characterised by interdependencies. The solution to the difficulties that occur during decision-making on reform proposals must not be sought in trying to accelerate processes or unweave the setting.