The Treaty on European Union unequivocally declares that the contracting parties are the Member States of the EU. However, a closer examination casts some doubt of the unitary status of Member States, or at least suggests that the concept requires nuancing. Whilst diversity, and to some extent differentiation, have been part and parcel of the European integration process since its inception, Redefining EU Membership proposes that, considering several developments, a new reflection on membership within the EU and on differentiation in and outside the EU is required.

The volume's contributions are organized around four aspects of the tensions faced by the concept of a unitary and formal EU membership: first, an examination of key policy areas which have already witnessed various forms of differentiated integration; second, an analysis of 'special statuses' within the EU; third, important examples of non-Member States in which EU law is applied (with and without their participation in the EU's decision-making process); and fourth, the situation of states under accession or secession procedures which obliges them to accept EU norms and policies even prior to/after formal membership. These analyses are complemented by a reflection on the concept of membership in itself.

In a context in which EU enlargement appears likely, the provided analysis reflects on the need to expand and introduce several kinds of membership to the EU in the future.