There are mechanisms in the Treaty on the Functioning of the European Union which allow the European Commission to bring legal proceedings against Member States which do not fulfil their obligations under the Treaty (see article 258 of TFEU) and also allowing one Member State to bring an action against another Member State (article 259). Similarly, Member States and EU institutions can bring proceedings against EU institutions for wrongful acts or wrongful failures to act. Member States and natural or legal persons are also able to challenge individual decisions and administrative acts in the Court of Justice of the European Union.
In practice, the Court of Justice of the European Union relies heavily on the national courts of Member States in order to help it handle the enormous workload of European Union law cases. The Commission actively encourages a greater role being taken by national courts. However, a national court will only have an opportunity to adjudicate on questions of EU law if the parties in the case before it are entitled to directly assert rights (or raise defences) based on EU law.
In EU law there is an important principle known as the doctrine of direct effect. This doctrine allows individuals and other legal persons (such as companies) to enforce their rights under EU law directly, as opposed to only Member States having the ability to do so. A provision is said to be of ‘vertical’ direct effect if a person can enforce it directly against a Member State. A provision is said to be of ‘horizontal’ direct effect if a person can enforce it directly against another EU citizen.
The doctrine of direct effect does not apply to all EU laws. It applies, broadly, only to those provisions which are sufficiently clear and precise that it would be possible for a national court to apply and enforce them in favour of a particular person. Treaty provisions and provisions of EU Regulations generally have direct effect as do some EU Decisions (generally decisions of the European Commission). Provisions of EU Directives can have vertical direct effect (but not horizontal direct effect), but only in the limited circumstance that the Member State concerned has not implemented the Directive into national law by the deadline date for implementation. Where the EU has entered into international agreements with other states, the provisions of those agreements may, in certain circumstances, have direct effect. Finally, an individual is sometimes entitled to invoke general principles of EU law, such as the principles of proportionality and non-discrimination, in proceedings before a national court in order to challenge the validity of an act of an European Union institution.
Where there is direct effect, and where EU law is inconsistent with the domestic law on the same subject, this raises a question over which law prevails (see here).