The aim of this article is to provide a brief overview of the debate on the subsidiarity principle. Subsidiarity is one of the most ambiguous and debated notions in law, and it definitively belongs to all the legal disciplines, from EU law to constitutional and administrative laws, passing through human rights law. The debate on this principle has been enriched recently by a number of articles and books focused on the new provisions concerning national parliaments included in the Lisbon Treaty. When dealing with subsidiarity, the impression, at the first glance, is that of a Cinderella principle because of its evanescent nature (rule or principle?) and of its difficult justiciability. This article suggests that just a strong change in the European Court of Justice's (ECJ's) case law might transform our Cinderella into a real constitutional principle. Despite the vast existing literature, I have decided to focus on a few, in my view, fundamental readings on this point by attempting to describe the noble design behind the introduction of the subsidiarity principle in EU law and the re-evaluation of the weight of this concept in the practice of courts. In doing so, I started from the recent contribution of Robert Schütze, in Chapter 5 of his book, From Dual to Cooperative Federalism: The Changing Structure of European Law, which defines subsidiarity as a 'political safeguard' of EU federalism. The first part of this article thus will be devoted to the analysis of this work. As will be evident after a few lines, I share the comparative approach chosen by Schütze, but despite this methodological convergence, our conclusions are different. In the second part of this article, I will try to show how the interpretation of subsidiarity, as followed by the ECJ, is misleading and, consequently, how the choice of the Lisbon Treaty to rely on the national parliaments as the most suitable watchdogs of subsidiarity has to be regarded as unsatisfactory.