Discussing what we may learn from thinking about the EU in federal terms represents a two-fold challenge. It is on the one hand a matter of establishing 'how federal' the EU is (the EU's federal challenge). On the other, the EU has federal features but is not a state, thus raising the question of whether federal theory and practice may have to be adapted to take proper account of the EU (the EU's challenge to federalism). The contributions to this collection supplement and extend existing scholarship through focusing on two important lines of inquiry. The first focuses on the relationship...