The aim of this paper is to describe the role of undercover investigators and confidants in the criminal proceeding. From the very name of the undercover investigator, it can be concluded that the undercover action is here the main subject, i.e. theso-called "Hidden police actions“. The paper describes their definitions, and the usage process is further elaborated. This research is conceived in a way that the historical inception of the disguised actions is first shown, and in order for such actions to be successful, special attention should be paid to planning and carrying out operations and selecting a undercover investigator. Both, undercover investigators and confidants, must accept their roles voluntarily in order to achieve the goal and it is relevant to mention that the identity of the undercover investigators and confidants must be strictly protected and kept secret from the public, so that such actions can be completed successfully. Furthermore, encouraging the suspect to commit the offense is also elaborated, as well as the importance of undercover investigators and confidants testimony. At the end of the paper, the consequences of the undercover investigators work are shown, and also evidenced by examples from practice. The use of undercover investigators and confidants belongs to special measures of inquiry, so it is clear that they will only be used in certain cases when they are really needed for the successful prosecution of a criminal proceeding.