This work was written with the aim of providing a as detailed as possible presentation of issues related to the duties of the investigator and, in general, investigative actions. The work is addressed to young legal professionals, judges, and lawyers, and aims to address the main issue of lack of familiarity with the main investigation process compared to courtroom procedures.

The procedure before the investigator is not public, making it quickly and easily accessible to everyone. Additionally, the main investigation sometimes occurs unexpectedly, that is, after the suspect's caught in the act, and consists of a sequence of actions that unfold over time. These characteristics favor the emergence of situations that cause embarrassment and the development of procedural practices that raise questions about their correctness.

Moreover, the actions of the investigator have a strongly technical-procedural nature, and a clerical omission of certain wording can result in the nullity of an investigative action or even a larger part of the investigation process.

This work brings together all issues that arise at each stage of the main investigation, from the receipt of the criminal file and the prosecutor's order to conduct the main investigation, to its formal completion and forwarding to the prosecutor. Concerns are presented based on judicial practice.

The purpose of this work is to serve as a detailed practical guide, from which especially the new investigator can draw support and, through its use, resolve doubts about how to organize the main investigation and the investigative actions they need to undertake.

From the defense side, the rights of the accused against the investigator are elaborated and defined, and the proper procedural route of each investigative action is pointed out, so that it is clear to the defense lawyer when an investigator’s action can be challenged and when not, guiding them and making this a necessary practical tool.