In the European Union, a state or EU institution can be sued by another state or institution in the Court of Justice when it is accused of violating Community law. However, if an individual wants to challenge the state on a Community law issue, they can do so in a national court. This principle, where both the Community and the states enforce the Community's law, is called dual vigilance.

Dual vigilance was not established in the EC Treaty, which only created the mechanisms for states and institutions to sue each other (see Article 226 EC and Article 227 EC). The Court of Justice actually created the concept in several landmark rulings, most notably Van Gend en Loos v. Nederlandse Administratie der Belastingen (Case 26/62). In that case, they maintained that the Community treaties imposed a direct effect on the individuals of the states, which could only be countered by a direct recourse to justice.

The dual vigilance concept was unprecedented in international law, and is still not found anywhere outside the EU. (In federations, which have similarly overlapping legal jurisdictions, federal and state laws are enforced by entirely separate courts.)

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