This chapter provides an outline of the constitutional theory of the federation and demonstrates how this theory provides an answer to the question of the legal and political nature of the EU. The federation is a discrete political form that transcends the ‘statist’ categories of Staatenbund (confederation) and Bundesstaat (federal state). These categories still obscure both federal theory and EU law scholarship, including the ‘sui generis’ thesis. The federation, or federal union, is founded on a federal compact (Bundesvertrag) between its Member States. This compact is a constitutional contract (Verfassungsvertrag) that gives birth to a new political entity, the Union, and transforms the Member States’ constitutions politically. The federation is a double political existence, consisting of the Union and the Member States, and it therefore has twin sources of authority. The federation is a genuine political order characterized by the internal absence, contestation or repression of sovereignty.