The Treaty of Lisbon introduced several innovations to the European Union#xE2;#xAC;"s institutional architecture. One of the most prominent of them was the inclusion of a so-called Early Warning System (EWS) for the principle of subsidiarity. Under this system national parliaments of the EU Member States are entrusted with the task of reviewing EU legislative proposals and of issuing, if they find that a proposal breaches the principle of subsidiarity, a letter called reasoned opinion. Incoming reasoned opinions are counted and weighed as votes and if the number of votes reaches certain thresholds then the initiator of the proposal, typically the European Commission, may withdraw, amend or maintain its proposal but must, in that case, justify its decision. The entry into force of the EWS marks a culmination of a sequence of Treaty reforms that addressed the principle of subsidiarity and the involvement of national parliaments in EU lawmaking. This book offers a comprehensive systematic analysis of the European Union#xE2;#xAC;"s new subsidiarity early warning system. The book includes both a detailed theoretical analysis of the EWS and an assessment of how under the system, national parliaments have responded to EU legislative proposals in reality. The book provides an overview of the historical developments of national parliamentary involvement in the EU and discusses the institutional aspects of the EWS including those which have been previously overlooked in the literature. The book continues with an empirical analysis of how national parliaments define subsidiarity for the purpose of their participation in the EWS incorporating the body of material resulting from the correspondence between national parliaments and the European Commission. The book also considers the broader implications of the EWS, including its relation with democracy and legitimacy and the possible involvement of the judiciary. Philipp Kiiver explores whether the EWS could function as a mechanism of legal accountability offering a partial remedy to the EU#xE2;#xAC;"s much-discussed accountability deficit.