The subsidiarity principle is of obvious importance in a federal legal order built on conferred competences. Here, the federal order refers to a dual-levelled form of governance, that is the central and the national that operate in constitutional plurality, or, in other words, the legal order of the European Union (the Union). In such mode of organization, the key issue is establishing and enforcing mechanisms whereby the efficiency of the federal structure is ensured while avoiding excessive centralization of regulatory power. The core ethos of the subsidiarity principle is thus to deal with the division of fields of competences between the Union and its Member States, or to ensure that the Union has not become a super-state, but indeed stays within its conferred competences. To put it bluntly, the principle of subsidiarity is ‘important in allaying fears about the “F” word (federalism)’. The ancient roots of the subsidiarity principle are well documented. In the EU legal context, subsidiarity was fi rst tied to the regulation of the environment but soon developed to respond to the loss of the right to veto decision-making procedures at the EU level following the Treaty of Maastricht. The role bestowed on subsidiarity was a judicially enforceable mechanism that prevents the excessive use of non-exclusive Union powers, pre-empting Member State legislative action. In this sense, the principle of subsidiarity serves as a substitute for the political safeguards protecting the Member States’ residual powers. In deploying such mechanisms, however, the EU has frequently been criticized for failing to protect the autonomy of national powers. Indeed, a recurring argument is that, if European federalism is to be safeguarded, new mechanisms should be sought. The argument to be advanced in this chapter is that two distinct approaches to monitoring subsidiarity, focused on procedural tests of competence, have the capability of providing a new approach to the judicial control of subsidiarity. More precisely, the Lisbon Treaty brings an increase in ex ante political control by empowering national parliaments to issue ‘early warning’ signals for breaches of the subsidiarity principle. The ex post legal control remains as a crucial complement, which, moreover, is strengthened by the new legal framework. In our view, this constitutional provision of the Lisbon Treaty, together with the parallel rise of the impact assessments used as an EU institutional policy, reflect a general move towards proceduralization. Th is shift in mood, or Zeitgeist, is starting to show in judicial review exercised by the Court of Justice of the European Union (ECJ) in subsidiarity cases that will be explored in this chapter. More precisely, this chapter starts by examining the procedural competence tests that are available in the post -Lisbon era. Our starting point is the competence test layered on top of Article 5 TEU; that is, the political safeguard vested with the national parliaments enabling them to warn the Commission of potential infringements of the subsidiarity principle. Describing first the functionality of such assessment, we then discuss the possible implications that these changes may lead to with regard to process-based review. Th e following part of this chapter discusses the second competence-based tool in monitoring the subsidiarity principle: the obligations imposed on the Commission to carry out impact assessment in the pre-legislative phase. Subsequently, the earlier case law involving the principle of subsidiarity is examined, followed by an analysis of the way in which the Court interprets subsidiarity cases in light of the two new process-based approaches. In the concluding part, the fi ndings in the chapter are summarized and the high aspirations for the new safeguards of federalism are confirmed.