In large-scale educational reforms, many actors play their roles. The diversity of contributions and lack of harmonization prove to be frequently found to cause educational reform failures. Many explanations for these failures focus on differences between the actors and on differences in their contributions to the reform process. In this article, we examine the effects of these differences and emphasize on the need to harmonize these contributions to the reform process. Contributions by several actors to a large-scale curriculum reform undertaken in the Netherlands in the 1990s are mapped for this purpose. This curriculum reform is part of a larger educational reform aimed to introduce a constructivist approach. Education is conceptualised as a social system, and educational reform as the manner in which this social system adapts to immanent and emmanent changes. The actors in the education system are distributed across functional subsystems. In the present analyses, teacher acting within a particular subsystem stands central. The results show adequate exchange and harmonization of the contributions from the different subsystems to be a necessary condition for successful educational reform. To achieve a good exchange and harmonization, the use of an Educational Impact Assessment is recommended.