Special investigative actions are one of the most important tools in the fight against organized crime, but they also represent measures of procedural coercion that significantly infringe on the rights and freedoms of individuals. In relation to the application of special investigative actions, incidental discovery are an exceptional means for uncovering and proving other criminal offenses and their perpetrators. Since incidental discovery in the application of special investigative actions provoke disputes among both legal theorists and practitioners, this final thesis seeks to clarify the issue through the consideration of international and domestic legal sources and the conditions for applying special investigative actions. It also provides an overview of the most commonly used special investigative actions, such as covert surveillance, the use of undercover agents and informants as methods of infiltration into criminal groups, and the acquisition of objects through special investigative actions. The thesis analyzes incidental discovery in the application of special investigative actions, focusing on the legal conditions that allow their use as evidence in proceedings where an incidental finding reveals a criminal offense or its perpetrator. It also examines incidental discovery in the practices of national courts and the European Court of Human Rights. Furthermore, the paper reviews final judgments of national courts to observe the application and significance of incidental discovery in domestic criminal law practice. Based on an analysis of the case law of the European Court of Human Rights, particularly concerning incidental discovery in the context of special investigative actions, the thesis critically examines the current normative framework of the Croatian Criminal Procedure Act. The Act is assessed in relation to the presented legal standards, and proposals for theoretically acceptable solutions are offered to ensure coherent future application of incidental discovery in police and judicial practice. The results of the research show that courts consistently adhere to legal provisions regarding special investigative actions and incidental discovery in connection with such actions. Courts increasingly reference the provisions of the European Convention on Human Rights and the case law of the European Court of Human Rights, specifically recognizing the erga omnes effect of these rulings. Finally, the research results provide a foundation for specific recommendations to achieve clearer and more precise legal definitions of incidental discovery, with the aim of eliminating uncertainties and gaps in judicial practice.