After a long period of absence, enlargement is back on the EU’s agenda. Following Russia’s full-scale invasion of Ukraine on 24 February 2022, it took only days for Ukraine to file its application to become member of the EU. All of a sudden, the EU is not only facing a brutal war in its immediate neighbourhood but also the prospect of being substantially transformed as we may be looking at a Union of no less than 35 members in the years to come. Some would argue that this will take a very long time and there is therefore little need to dive into discussions about how the EU should operate, once it has grown. Others refer to the notion of ‘absorption capacity’ and emphasise that unless the EU changes its own policies, budget, decision-making rules and institutional set-up there is a risk that it will simply stop working following enlargement. The aim of this volume is to provide scholarly perspectives on how the EU should – or should not – change in order to enlarge further. In the first chapter, Göran von Sydow and Valentin Kreilinger set the scene by explaining what they mean by ‘Fit for 35’ and why we should care about reforming EU politics and institutions for an enlarged Union. They raise crosscutting issues that are particularly relevant to the volume and contextualise the topic within the current political and academic debate and the evolution of European integration.The second chapter, by Frank Schimmelfennig, begins by pointing out that the Russian invasion of Ukraine has put enlargement back at the top of the EU agenda, and that this has confronted the EU with a dilemma. Whereas the geopolitical situation creates the need to accelerate the accession process, neither the EU nor the candidate countries are sufficiently prepared. Schimmelfennig proposes that differentiated integration would help to resolve the dilemma. Differentiated integration would facilitate the enlargement process by initially excluding new member states from those policy areas that would be most negatively affected by the expansion of the membership. It would give the EU and the new member states additional time and incentives for reform without blocking enlargement. The chapter reviews the rationale and record of differentiated integration in EU enlargement and claims that differentiated accession is established practice and would likely be more pronounced and durable in any future enlargement. Schimmelfennig further discusses and elaborates existing proposals for ‘staged accession’ and considers potential pitfalls and objections to differentiated enlargement. He concludes that differentiated membership is more likely to be feasible and acceptable to both members and candidates than quick institutional reforms.8 Fit for 35? Reforming the Politics and Institutions of the EU for an Enlarged Union SIEPS 2023:2opThe volume continues with an essay by Yves Mény (chapter 3). According to Mény, the EU is again confronted with a crucial but not new dilemma: growing to limits in order to address the demands of the states which are not yet members of the European club or taking the risk to rock the boat by being unable to adapt means to goals and ambitions. This chapter explores the dilemma and discusses the possible options which could reconcile ambition and realism, in other words enlarging and deepening at the same time. Mény shows some skepticism about the desire and capacity of the 27 present member states to adapt given the heterogeneity of visions and interests among them. All possible options have already been aired and debated. What is lacking is the political will on the part of the individual states taken as a political community. It is as if the member states were renouncing the exercise of their collective capacity unless forced to do so by circumstances and historical developments.The fourth chapter, by Sonja Puntscher Riekmann, turns to the aim of ‘fitness’ and the goals of the EU. Fitness, in terms of a political community is a variable dependent on purpose. This truism applies to the European Union as much as to all polities. While enlarging the Union in Eastern Europe and the Western Balkans is a tall order that would considerably increase its territorial and demographic size as much as its socio-economic and cultural diversity, the debate about ‘what are we together for?’ is far from concluded. Russia’s war against Ukraine accentuates the need to discuss what European elites have in mind when they convey ideas of ‘sovereign Europe’ (Macron/Scholz), of ‘a geopolitical Commission’ (von der Leyen) or of ‘speaking the language of power’ (Borrell). With democracy being one fundamental value of the Union, such discourse needs to draw on citizens’ views on the future of the Union. Puntscher Riekmann argues that citizens’ expectations about security and prosperity as European public goods run high, while the foresight capacity and strategic thinking of elites leaves much to be desired. However, the Union has difficulties even in developing a shared definition of problems and crises. Indeed, definitions often emerge from an ad hoc and cumbersome search for compromise between divergent national interpretations and interests. Further enlargement will also add complexity regarding the daunting geopolitical challenges Europe faces. Hence Puntscher Riekmann concludes that if sovereignty is to become a meaningful concept in EU affairs, it needs clarification as to the nature of the sovereign, the tools by which that sovereignty is to be exercised, and citizens’ support. For that matter, treaty reform before enlargement is a worthwhile risk to take.In the next contribution (chapter 5), Tanja Börzel takes a critical perspective on treaty reform and argues that it would not make the EU fit for enlargement. Putin’s war of aggression against Ukraine has boosted demands for the deepening of European integration. Institutional reforms are deemed indispensable to prepare the EU for the accession of the Western Balkans and Ukraine as well as SIEPS 2023:2op Fit for 35? Reforming the Politics and Institutions of the EU for an Enlarged Union 9to build the EU’s strategic autonomy in security and defence. But irrespective of the degree of pooling and delegation of national sovereignty deemed necessary to make the EU fit for 35 members, changing the treaties would take time. Member states not only have to agree on reforms, they also have to ratify them, which entails a popular referendum in some cases. Börzel points out that amid weak public support for enlargement, seeking to deepen the EU could weaken, rather than strengthen, the EU’s capacity to widen. According to her, the key challenge for the EU is to find a way to balance rule of law conditionality against the credibility of accession and geopolitical pressures.In the sixth chapter, Sergio Fabbrini challenges the view (shared by scholars and politicians) that the EU is not a political system, but rather the contingent outcome of an evolving process that will lead to the aggregation, although differentiated, of all the states of the European continent. This view has outlived the crises of the last fifteen years (Brexit among them) and has been further strengthened by Russia’s invasion of Ukraine, with the related pressure to enlarge the EU to that country, to Moldova and possibly Georgia, as well as to the six countries of the Western Balkans. The EU, Fabbrini argues, is indeed a political system; one with a dual governance structure, supranational and intergovernmental. The entry of core state powers onto the EU agenda with Maastricht and the enlargements of the 1990s and 2000s has dramatically strengthened the latter to the detriment of the former. Those processes in fact triggered divisions on the role of national sovereignty that only the European Council could manage, bending the EU in direction of an international organization. Is this development coherent with the promise of an ‘ever closer union’? Further enlargement would require a change of paradigm, from a multi-speed EU to a multi-tier Europe, making thus Europe fit for herself.In the final chapter Göran von Sydow and Valentin Kreilinger try to connect the dots by means of some concluding remarks. They look backwards to the 1990s and forwards to the remainder of this decade, for indications of what the problems are – and the opportunities for solving them.These contributions provide different perspectives and make different prescriptions about if and how the EU should change. While many point to the difficulty of a Union of 27 engaging in tiring discussions about internal issues and burdensome negotiations about, for instance, treaty change, others argue that in order for the EU to function such reflections are necessary. In this volume there are no common conclusions. Instead, the authors provide distinct and thoughtful perspectives on what could well be a defining process for the EU.