Abstract The concept of subsidiarity was a notable addition to the Maastricht Treaty when it was first introduced. It continues to be of political and legal significance in the post-Lisbon world. This article considers subsidiarity from a political and legal perspective. It analyses the diverse rationales behind its inclusion in the Treaty, and the reasons why it has proven difficult to realize the objectives of subsidiarity. The focus then shifts to the legal dimension and considers the role played by the EU courts in monitoring subsidiarity and suggestions that the EU courts should review subsidiarity through a form of competence-proportionality control.