This study deals with the principle of subsidiarity in European law. Pivotal point is the Treaty establishing a Constitution for Europe signed in Rome in October 2004, though not ratified in all member States. In chapter 1 of the book the author first examines the question whether the principle of subsidiarity had already been implicitly embodied in earlier European legislation. The authors answer is essentially in the negative. The author then comments on the principle of subsidiarity as laid down in Article 5, paragraph 2, of the EC Treaty. Chapter 2 centres on the discussion on a better implementation of this principle during the negotiations on the Rome Treaty of 2004. Alternative drafts of a constitution for Europe are also discussed. Chapter 3 deals with the provisions of the Treaty of 2004 concerning subsidiarity, especially Article I-11, paragraph 3, thereof and the Protocol on the application of the principles of subsidiarity and proportionality. According to the author, the Treaty aims less at a new definition of subsidiarity as a principle of law, but focuses rather on a concept of procedural protection. Moreover, the principle of subsidiarity is effectively guaranteed only if it is strictly interpreted as a principle of law, leaving aside the manifold sources of subsidiarity in the history of ideas.