This thesis investigates how regional cooperation can enhance the implementation of area development in the Netherlands by comparing the cases of Leidsche Rijn and Rijnenburg. These two developments represent different planning regimes: Leidsche Rijn emerged during the centrally coordinated VINEX era, while Rijnenburg unfolds in a fragmented, decentralized governance context shaped by contemporary NOVEX policy. The study adopts a qualitative methodology combining document analysis and stakeholder interviews, guided by an analytical framework focused on conditions, implementation, and steering. The ϐindings show that regional cooperation, deϐined as a governance approach in which public and private actors coordinate across administrative boundaries to align goals and resources, was crucial to the success of Leidsche Rijn. In contrast, Rijnenburg struggles to establish such cooperation due to unclear institutional roles, fragmented land ownership, and a lack of shared narrative and steering authority. A key barrier is the unresolved development of the Merwede line, a critical infrastructure condition. Moreover, while national involvement is needed, its precise role remains undeϐined. The thesis concludes that regional cooperation is essential but not automatically achieved; it must be actively conϐigured over time. The analytical framework proves useful for assessing regional governance but requires further testing in other cases to validate its broader applicability.