This is an open access title available under the terms of a CC BY-NC-ND 4.0 licence. It is free to read on the Oxford Academic platform and offered as a free PDF download from OUP and selected open access locations. What role do procedural rules really play in shaping the behaviour of legislative actors? In theory, competences and procedures provide an accepted framework within which ministers, members of parliament, and senators draft and bargain over legislative proposals. Contested Competences in the European Union challenges this perception of legislative operation, revealing that the question of who gets to make decisions, and in accordance with which procedure, is often just as contested as the substantive content of legislation. While institutional rules constrain actors' behaviour, they are not as fixed as their 'constitutional' nature might suggest. Instead, competences and procedures are regularly adapted through legislative practice in response to political pressures and contestation. Drawing on archival and statistical evidence, Contested Competences in the European Union presents a comprehensive theoretical, empirical, and historical account of the phenomenon of competence contestation across key policy areas. It reveals that the European Commission, Parliament, and Council frequently clash over legal competences, strategically bending procedures to suit their political goals. Despite impeding decision-making efficiency and diverting focus away from policy problems, this constant tug-of-war over rules has become an enduring part of how the EU works - and is unlikely to disappear any time soon.