Abstract Though best known for its coverage of key policy areas, the Maastricht Treaty also introduced important institutional reforms. Building on the Single European Act, these pertained mainly to strengthening the legislative role of the European Parliament and extending the scope of qualified majority voting to more policy areas. Treaty-based institutional reform continued in the post-Maastricht period. Due to the twin challenges facing the EU in the aftermath of Maastricht — Central and Eastern European enlargement, and the gaping ‘democratic deficit’ — the focus of such reform shifted to the modalities of qualified majority voting and the size and composition of the Commission. The highly contentious nature of these issues thwarted effective institutional reform in the 1990s, overshadowed the work of the 2002–2003 Constitutional Convention, and dominated the intergovernmental conferences preceding the failed Constitutional Treaty and the subsequent Lisbon Treaty.