The article explores the interplay between national courts and European Union institutions in the context of competition law enforcement of rules enshrined in Articles 101 and 102 TFEU. Cooperation between national competition authorities and national courts on one hand and EU institutions on the other hand is crucial for ensuring the effective and consistent application of competition rules. The article deals with the mechanisms of this cooperation. It examines the role of the European Commission and the Court of Justice of the European Union (CJEU) in providing guidance to national competition authorities and national courts. The article underscores the importance of a cohesive approach to competition law enforcement that is applied by national institutions of the EU member states. The concept and meaning of legal security in criminal law is well defined by Professor Jadwiga Potrzeszcz who states that "legal security is a state achieved by law established in general, and in particular by means of criminal law, in which human life’s goods and interests are protected in the most comprehensive and effective possible manner (and) harmonizes properly with the most important functions of criminal law, namely with a protective function and a guarantee function." (Potrzeszcz, 2018, p. 301) In accordance with the aforementioned definition, the author aims to introduce Criminal Codes that were in effect in the territory of present-day Slovakia, shedding light on whether they were guarantees of legal security in the state or not, i.e., whether they protected "human life’s goods and interests in the most comprehensive and effective possible manner."