This incisive book examines the legitimacy of the EU to enact direct tax law for businesses in order to avoid detriment to the internal market. François Barreau discusses the claim that the EU is a sovereign order, theorising that it has a right to a shared competence in the field of direct business taxation. Barreau explores both the definition and methodology of sovereignty, as well as their impact on the political sovereignty of the EU, demonstrating that the EU does not have the adequate means to achieve its sovereign objectives in the field of direct taxation. Building on this the book also proposes the use of shared sovereignty, as a theory of unity and relations based on Leibniz' monad, to restore coherence between the EU and the Member States and to give the EU a shared competence in the field of direct taxation of businesses. This insightful book is a key resource for scholars and students across numerous legal fields, including general EU law, European tax law and constitutional and administrative law. It is also beneficial to policymakers and European institutions seeking more information about the EU's order and taxation powers.