This PhD thesis studies the role of national parliaments in the policing of the EU subsidiarity principle. The Treaty of Lisbon enshrines the Early Warning System (EWS) in Protocol No. 2, according to which national parliaments may review Commission proposals for compatibility with the subsidiarity principle expressed in Article 5(3) TEU. On the basis of the number of reasoned opinions submitted, which count as votes, national parliaments may trigger either a ‘yellow’ or an ‘orange’ card, each of which entails different consequences for the Commission draft act in question. The purpose of this thesis is to analyse the functioning of the EWS and to explore why national parliaments participate in this mechanism. To achieve this task, this thesis analyses the reasoned opinions issued under the EWS. Hence, this thesis firstly conducts a case study of the Commission proposal on the establishment of the European Public Prosecutor’s Office which triggered a ‘yellow card’. This example shows that national parliaments tend to conduct a broad scrutiny of Commission proposals, which includes aspects other than the subsidiarity of the proposal: its legal basis, the competence of the EU to act, its proportionality and its substance. This practice of national parliaments is evaluated according to a textual, structural and functional interpretation of the EU Treaties, and as a result, a narrow subsidiarity test is suggested for the purpose of the EWS. Thereafter, the thesis explores the national procedures of ex ante (EWS) and ex post (action before the ECJ) scrutiny. In addition, national debates are studied in order to analyse the relationship between national legislatives and executives, between parliamentary majorities and opposition, as well as the reflection of regional interests. This detailed study of debates also points to the first reasons for the participation of national parliaments in the EWS: the protection of idiosyncratic national interests and the restriction of EU redistributive policies. Further reasons for national parliaments’ participation in the EWS are indicated on the basis of two case studies, dealing with the Monti II regulation (competence), and the Tobacco Products Directive (‘delegated legislation’). These suggest that the EWS is used by national parliaments to increase their impact in the EU legislative process. The last case study of this thesis – the ‘Women on Boards’ proposal – ponders the application of the EWS to ‘genuine’ fundamental rights proposals, showing that the subsidiarity tests at stake here are focused to a much greater extent on a political willingness to protect universal values, rather than on efficiency. The thesis concludes by discussing whether the EWS enhances the EU’s democratic legitimacy and decreases the EU’s competence creep, which were the leading ideas behind the introduction of the Protocol No. 2 mechanism. It is pointed out that, although the impact of national parliaments on EU policy-making is uneasy to measure, some of the criticism of national parliaments is taken on board by the EU legislator. Because the ‘competence creep’ of the EU is rather limited, it also does not demand a great deal of involvement on the part of national parliaments.