The role of taxation within the European Union has risen (and is rising) over the years due to the establishment of the Economic and Monetary Union and the economic/financial and COVID crises. It is well established that taxation is characterized as an obstacle for the functioning of the internal market and the interpretation of the fundamental freedoms has a direct impact on the design of the domestic tax systems. The first part of this article addresses the clash between the supranational economic freedoms and the domestic tax systems in the light of the federal experience of “two levels of government”. The second part, differently, sheds light on the role of taxation within the Economic and Monetary Union. After having enucleated a principle of solidarity according to article 122(2) of the Treaty on the Functioning of the European Union, the article inquires the foundations and the limits of the European Union’s authority to tax. In particular, it aims at demonstrating that the financial assistance to the Member States in exceptional circumstances should be funded through the EU budget and EU taxes.