It is widely recognised that the enlargement of the European Union to ten new member states in May 2004 represents great institutional and political challenges both for the u0091oldu0092 and u0091newu0092 member states. The EU has tried already three times to prepare its decision-making mechanisms for the near doubling of its membership: with the Treaties of Amsterdam, Nice and the Constitutional Treaty. However, the effectiveness of the EUu0092s governance is also endangered if the member states are not capable of implementing EU rules correctly. The post-enlargement situation is characterised by a huge gap between the capacity of the new member states to implement EU rules and the growing complexity of the post-decisional phase. This study analyses the risks that this situation entails for the governance of the enlarged Union. It also proposes necessary reforms to ensure the effectiveness of this governance in the future.