One of the central tenets of adaptive management is polycentric governance. Yet, despite the popularity of the concept, few detailed case studies of polycentric governance systems exist. In this paper, we aim to partly fill this gap. We describe water management between the years 1000 and 1953 on the Dutch island of IJsselmonde in the Netherlands near Rotterdam, and then use this case to reflect on the theory of polycentric governance. Despite the small size of the island, water management on IJsselmonde was the responsibility of no fewer than 31 local jurisdictions and some 65 polders. In addition, some supra-local arrangements were made, such as joint supervision of dikes. According to the theory, such a polycentric system should have many advantages over more centralized management systems, and indeed there is some evidence of this. Yet, there is also evidence of a disadvantage that is not mentioned in the literature: petrification. IJsselmonde's water management system was often slow to adapt to changing conditions, and at times it provided an answer to yesterday's challenges rather than today's. We conclude that the theory of polycentric governance needs to be developed further because it now lumps together too many different systems under the heading of polycentric governance. This calls for more longitudinal case studies on the development and effectiveness of individual polycentric governance systems within their changing context.