The aim of this chapter is to illustrate how the principle of proportionality as derived from the legal orders of the Member States has been incorporated in the legal order of the European Union as well as codified in the Treaties. The chapter revolves around two key arguments. It emphasises, first, that the principle of proportionality has been used as a criterion for controlling the legality of the acts of the Union and of those of the Member States when implementing Union law. Secondly, it is argued that, more recently, the same principle has been used by the Court of Justice of the European Union as a useful instrument towards ensuring an even stronger coordination with national judges.