The formation of the European Union was based on the idea of building a politically stable and economically prosperous Europe free from wars, with the aim of overcoming the horrific aftermath of World War II on the continent. By creating this integration and gradually incorporating an increasing number of countries, the member states agreed to and encouraged the establishment of institutions to which they would transfer part of their sovereignty and national powers in order to achieve a common mission: mutual cooperation and the strengthening of collective potential. As the founding treaties of the Community (later the Union) were drafted and amended, almost every amendment assigned new, expanded powers to the institutions, and this supranational integration assumed the right to regulate an increasing number of areas. The expansion of the powers of the Union and its institutions resulted in the creation of various policies and the adoption of legislation to implement these policies thus encroaching on the rights and obligations of not only member states but also their citizens as subjects of this legal system. In this sense, it is particularly important to ensure that the actions of the Union’s institutions are based on legislation and remain within the limits of their assigned powers. To guarantee this, supervision mechanisms were established as a system of protection against unlawful actions, one of which is the supervision by the Court of Justice of the European Union. This paper will present the legal system of the European Union, its institutions, the development of the Court of Justice of the European Union through the founding treaties, the legal framework of its activities, its competences, and the elements and procedures for protecting legality and expediency before the Court in cases of unlawful actions by the Union’s institutions, bodies, offices, and agencies.