The author of this dissertation acknowledges the concept of source of law is a general theoretical concept that pretends to be applied to any judicial order. It is by no means a general theory, but implies that every legal order has its sources and that the determination of sources in individual legal orders is a matter of positive law. Court rulings play a key role in the legal system of the Republic of Croatia, as well as in many other legal systems in the world. Court decisions in Croatia are handed down at all levels of courts, starting with municipal and county courts and ending with the Supreme Court of the Republic of Croatia. Decisions of higher courts have greater legal weight and are binding on lower courts. Court verdicts are therefore a key element of legal certainty and legal predictability in Croatia and play a vital role in the realization and protection of the rights of citizens and legal entities. The sources of law of the European Union are the fundamental basis of its legal system, making it unique and complex. These sources of law include primary law, secondary law and supplementary law, while there are also additional sources of law such as case law, international law and general principles of European Union law. In addition, the European Union has its counterpart to Narodni Novine as the official newspaper of the Republic of Croatia, which is EUROLEX. The author concludes that the legal system of the European Union is complex and integrated, combining elements of different legal traditions. Court decisions, especially those made by the Court of Justice of the European Union, play a key role in shaping and applying EU law and ensure consistent interpretation and implementation of EU legal acts in all member states.