The 2004 “Big Bang” enlargement was a powerful reminder that European integration is an instrument for peace and not just prosperity. The pace of that enlargement depended more on the requirements for stability than on the transformation of the candidate countries. It was also a reminder of the importance of forward-looking analysis. Candidates might meet the criteria for membership at the time of accession, but that is no guarantee that they will develop in ways that continue to reflect those criteria once they have gained entry. Finally, it was a reminder that enlargement changes the experience of membership for all member states and not just for those countries that gain entry. A larger Union requires greater self-discipline to hold down congestion in decision-making and greater multilateral surveillance to prevent the actions of one member state from undermining the benefits of membership for the rest. These reminders are important lessons in planning the European Union’s next historic enlargement. The next enlargement will follow a pace set by security considerations more than the transformative power of the accession process. It will depend on a robust analysis of convergence together with contingency planning for any staged accession. And it will require commitment from existing member states as well as candidate countries to what will become a very different European Union. This next enlargement will be challenging for all parts of Europe. Nevertheless, it is better than the alternative of no enlargement or an accession process with no credible endgame.