Introduction: the article deals with key aspects of the differentiation of procedural forms of criminal proceedings. The focus is on the current state, challenges and perspectives to improve preliminary investigation process forms. Materials and Methods: the methodological base of the given research are said to be a dialectical method of scientific knowledge, historical, logical, comparative legal, statistical, concrete sociological methods as well as methods of observation and other particular methods of research of legal phenomena. Results: the differentiation of the forms of criminal proceedings being an initially theoretical matter is of great importance for efficient implementation of criminal prosecution and solution of the criminal procedure tasks at all its stages. Since the Code of Criminal Procedure has been adopted and up to nowadays the legislator making attempts to differentiate the forms of pre-trial proceedings does not fully take into account the scientific experience in this field and the needs of practice that lead to disappointing results. Differentiated procedural forms of investigation do not meet the requirements to solve quickly and efficiently the criminal procedure tasks. Discussion and Conclusions: the authors based on the study of the grounds, goals and objectives of the differentiation of pre-trial proceedings as well as the results of the use of existing forms conclude about the unequivocal necessity to modernize a current preliminary investigation taking into account the “work on the mistakes” made by the legislator in legal regulation during the period of validity of the Code of Criminal Procedure of 2001.