Differentiation is a principle that has applied to the polity and policies of the European Union since its creation. It has been reinforced in the last 20 years as a consequence both of successive enlargements and of a broadening of the EU policy agenda. The British renegotiation process and the UK referendum scheduled for 23 June 2016 constitute a further stage in the debate on differentiation. For the first time a 'no' vote in a national referendum could mean that disintegration rather than differentiation is the preferred choice for an EU member state.The leaders of the EU member states, as well as scholars and analysts of the EU, have difficulty dealing with the concept of disintegration. For this reason, it is hardly surprising that most of them would like to see the UK referendum ending with a 'yes' vote. Yet this position is not necessarily shared by all segments of European public opinion. Those who have strong Eurosceptic concerns would like to use a Brexit as leverage in their own contests with the EU.This article considers the future of differentiation in the light of the debate on Brexit. To this end, the article has been divided into four parts. We start by examining the concept of differentiation in the EU from a theoretical perspective. In the second part, we analyse different national practices of differentiation in the member states. In the third part, we try to understand the reactions of the member states towards the British demand for renegotiation in order to avoid a Brexit, before drawing some conclusions in a fourth part. Differentiation: more integration or more fragmentation?Differentiation and integration are intricately linked. Differentiation may be defined as the process that allows some EU member states to go further in the integration process, while allowing others to opt not to do so. It therefore runs counter to the assumption of 'ever closer union' among the member states.The debate over differentiation revolves around the distribution of competences across all levels of government-local, regional, national and European. To be more precise, the question of differentiated integration arises as part of a debate * The authors would like to thank Douglas Webber for his comments on an earlier draft.INTA92_3_ChopinLequesne.indd 1

There is a proliferation of works on the new European External Action Service (EEAS). Most of these approach the EEAS through a rationalist framework, assessing how a new institution can solve long-term questions of EU foreign policy-making to ensure consistency and coherence while reducing transaction costs between actors (both supranational and national) in a multilevel governance structure. This paper takes a different direction. Using 30 interviews with officials from the EEAS, the European Commission, and national ministries of foreign affairs, conducted between 2010 and 2013, it shows how the study of practice aids understanding of the nature of the EEAS. As a new institution, the EEAS lends itself particularly well to practice-based study because new institutions must develop new practices.The first section of the article defines the notion of practice and shows the importance of historicizing the struggles around practices in understanding the creation of the EEAS. The second section demonstrates how agents' practices shape professional cultures within the EEAS. The third section highlights the relationship between practices and rule-making. Going over the EEAS as a case study, the conclusion focuses on the importance of analysing actors' practices for understanding the current evolution of diplomacy and international relations in general.

Paradiplomacy, federalism and international negotiation are increasingly prevalent phenomena that require more theoretical attention. Successful mobilization of non-central governments has increased their relevance on the international stage. The rise of paradiplomacy complicates conditions for both international negotiation and the formulation of foreign policy in federal regimes. Westphalian state diplomacy is finding it increasingly difficult to cope with the proliferation of ad hoc and informal arrangements that bind non-central governments. The international arena is inhabited by an ever larger number of players that sometimes have significant autonomy from the central state.

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