This chapter examines the relationship between federalism and fiscal authorityAuthority in the European Union, focusing on how the allocation of fiscal powers affects political autonomyAutonomy and the constitutional balance of federalFederal systems. It outlines the principles of fiscal federalismFiscal federalism, including taxation, spending, redistribution, and liability, and explores the tensions between state autonomy and federal coordination. The analysis then turns to the EU, showing how the Maastricht model and subsequent euro-crisisCrisis reforms redefined the EU’s fiscal federalism through enhanced supervisionSupervision, conditional financial assistanceAssistance, and new sanctioning procedures. While these reforms increased EU oversight and influence, Member States remain the ultimate decision-makers, particularly in sanctioning and financial liability. The chapter concludes that EU economic governance illustrates both the necessity and risks of federal intervention in fiscal matters: it safeguards stabilityStability but also challenges the principles of federal integrityIntegrity, loyaltyLoyalty, and adjudication, thereby testing the federal legality of the EU’s constitutional order.