The purpose of this study is to participate in the public scientific dialogue regarding the character of European Union law and to address certain critical issues related to the new categories of acts, which concern the legal nature of acts when applying secondary law within each member state.

Essentially, this study represents the first comprehensive analysis of the new categories of Union secondary law acts introduced by the Lisbon Treaty. These are the fundamental categories of legislative and non-legislative acts, as well as the more specific categories of delegated, implementing, and non-legislative acts that are directly based on the Treaties.

The provisions of the Lisbon Treaty condition the category of each act on the procedure by which it is issued, whether the procedure is foreseen in the Treaties or has been established by secondary law acts, the content of which is determined in the Treaties.

Through the reliance on the issuing procedure of each act, the new categories connect the acts of secondary law with the institutions that issue them, a feature that was absent from the Union's legal system before the last revision.

The study highlights the need to integrate the new categories of acts of secondary law into the Union's legal system as it had been shaped up to the Lisbon Treaty and to seek the changes they bring to this system, taking into account that the typology of legally binding acts of secondary law (regulations, directives, decisions) as defined by the Founding Treaties remains in force.

For this reason, the first chapter of the study approaches Union law from the perspective of the theory of legal systems, and the second identifies certain key systematic characteristics of Union law.

The third chapter examines the specific rules governing the establishment of acts of Union secondary law, which have been referred to by the Court as "the legislative system of the Treaties," as these rules had been formed before the last revision, and the fourth chapter analyzes the transition from the previous regime to the innovations of the Lisbon Treaty through the Assembly for the Future of Europe and the Constitutional Treaty.

The fifth and sixth chapters analyze the new categories of acts based on the case law of the Court of the European Union, which has been intensive in this area in recent years, and in the seventh chapter, from the perspective of individuals, certain specific issues of judicial review and protection of fundamental rights linked to the new categories of acts are addressed.

In the concluding remarks, the main issue of the hierarchy of secondary law rules and the contribution of the new categories of acts to it is re-examined.