Article 17 of the Treaty on European Union (TEU) states that the European Commission is the guardian of the EU treaties. It thus has the task of monitoring the application of EU primary and secondary law and ensuring its uniform application throughout the EU. It gathers information to monitor EU countries’ compliance.
Where an EU country fails to comply with EU law, the Commission may send it a ‘letter of formal notice’, which allows the EU country to submit its observations. If the Commission is still of the opinion that the violation of EU rules persists, it sends a reasoned opinion to the EU country. If this opinion remains without effect, the Commission may bring the case before the Court of Justice of the European Union (under Article 258 of the Treaty on the Functioning of the European Union (TFEU)).
If the EU country fails to observe the judgment, the Court may impose a financial penalty on the country in question following a second Court procedure (Article 260(2) TFEU). In certain specific cases, the Court may impose financial sanctions already when passing its first judgment under Article 258 TFEU (Article 260(3) TFEU).
Any individual or entity may also lodge a complaint with the Commission if a measure or an administrative practice in an EU country appears to violate EU rules. However, it is only the Commission that may initiate the procedure under Articles 258 and 260 TFEU.
Source: EUR Lex Glossary