The principle of mutual trust, whose fundamental importance is recognized by the CJEU, is not mentioned in the Treaties, but nonetheless, it plays an essential role for the EU integration process and has become a structural principle of the EU law. In addition to its role as a basis for a large set of EU rules in the areas such as the internal market and the area of freedom, security and justice, this principle is also closely related to the EU founding values including the rule of law. Having in mind that is not a “blind trust” but an assumption, it is applied through ensuring compliance with the Union law for which both the Member States and the European Commission share responsibility, inter alia, by means of the infringement procedure. Under Article 259 TFEU, Member States are also entitled to bring a direct action against another Member State for an alleged infringement of an obligation under the Treaties. However, it is extremely rare for a Member State to take action upon the Article 259 TFEU and its potential remains untapped till now. This contribution aims to answer why do Member States are inactive in terms of invoking the infringement procedure. It argues that infringement procedure initiated by a Member State against another Member State should not be perceived as a violation of the mutual trust between them but as a tool to uphold the mutual trust and to protect the Union’s founding values, including the rule of law.

The doctrine of supremacy is essential to the uniformity of the EU legal edifice. It had no formal basis in the Treaty Law but was developed by the Court of Justice of the EU by means of its conception of the "new legal order" (Costa v ENEL). Therefore, the corollary of sovereignty of the EU legal order is the supremacy of EU law: any norm of EU law takes precedence over any provision of national law. From the CJEU's perspective, supremacy entails duty for the national courts to 'set aside' any conflicting national norm when an EU rule applies in a given case. Ultimately, the acceptance and application of the supremacy of EU law are dependent on the Member States. Despite its invention, acceptance of the doctrine of supremacy has been the main challenge within the overall integration process. Recent ruling from the German Constitutional Court (the Bundesverfassungsgericht) on the legality of the European Central Bank's Programme marked that the supremacy issue cannot be put 'ad acta' and still continues to be surrounded with ambiguity and controversy against its unconditional acceptance as the CJEU requires. This paper summarizes the most remarkable aspects of the foundations of the supremacy doctrine and the conceptual basis on which the Member States accord supremacy to EU law, as well as its scope and limits. All this is necessary in order to be able to determine the perspectives for ensuring the supremacy of EU law, while highlighting its importance for the future of the European integration.