This book offers a comprehensive systematic analysis of the European Union’s Early Warning System for subsidiarity, which was introduced by the Treaty of Lisbon. The book includes both a detailed theoretical analysis of the Early Warning System as well as an assessment of how national parliaments have responded to EU legislative proposals in reality. The author explores whether the System could function as a mechanism of legal accountability offering a partial remedy to the European Union’s much-discussed accountability deficit. The book provides an overview of the historical developments of national parliamentary involvement in the EU and also considers the broader implications of the Early Warning System, including its relationship to democracy and legitimacy.