Based on an understanding of the procedural function of the investigator as criminal prosecution in pre-trial proceedings, the use of the criminal procedural legislation of Russia and a number of other states, and the resulting specificity of law enforcement activities, the work examines in depth the theoretical foundations and problems of the theory of its activities. In particular, questions about the essence of criminal prosecution, traces - the only “material” of its implementation, the formation of evidence, the limits of the investigator’s permissible discretion, the relationship between the accusatory position and the accusatory bias when carrying out criminal prosecution and a number of other relevant problems and dilemmas relevant to the topic of the work are considered. From positions substantiated on these issues with the active involvement of materials from published and unpublished investigative and judicial practice, the pragmatic foundations of the investigator’s activities in pre-trial proceedings in a criminal case are also explored in depth: the use of special knowledge, investigative actions and methods for investigating crimes of certain types and categories. The publication is intended for theorists and practitioners of criminal proceedings, graduate students and university students of all levels of higher legal education.