The diploma thesis investigates the issue of special evidentiary actions within the Croatian criminal procedure. The context of the work is based on the assumption for undertaking and the modalities of implementation of each special evidentiary action with an emphasis on the subsidiarity of their application and respect for the principle of proportionality. The material assumptions and the undertaking of special evidentiary actions were analyzed, especially the grounds for suspicion, the catalog of criminal offenses for which special evidentiary actions can be ordered, and the evidentiary difficulties for conducting investigations that justify their application (principle of proportionality). The formal assumptions were analyzed in detail and critically reviewed, with special emphasis on the state attorney's request and the investigative judge's written and explained orders. In the context of formal assumptions, the provisions related to the time limit of their application were also analyzed. The roles of the bodies participating in the process of obtaining orders for the implementation of special evidentiary actions are explained. In particular, the triangle of cooperation between the state attorney who requests the issuance of an order for the determination of special evidentiary actions, the police, who conducts special evidentiary actions, and the investigating judge, who orders them and carries out the judicial control over their implementation, is described. A systematic treatment of the way of undertaking individual special evidentiary actions was succinctly approached while touching on the specifics that appear in each individual action. Through a practical example from practice, the difficulties in preserving the secrecy of the measures are illustrated, because due to the specificity of the structure of the organizations participating in the implementation of the measures, a large number of officials within the repressive system become familiar with information that should be secret. The goals of this paper are not just to explain the institute of special evidentiary actions, but to provide a practical understanding of their conditions of application and methods of implementation. The paper emphasizes the importance of special evidentiary actions as a necessary tool of modern criminal justice that can significantly contribute to the detection and prosecution of perpetrators of serious crimes, which are often characterized by conspiratorial and secret actions.