Introduction. The legal, in particular, the criminal law direction of the comparative method application in science is currently an actively developing teaching in the system of domestic criminal law science. Because of this, and in the light of the fundamental shifts in world politics taking place in recent years, entailing corresponding changes in the system of international law, scientific research in the direction chosen by the authors is being updated. Materials and methods. The research is of a subject-scientific nature and is carried out, in general, on the basis of the study of the criminal legislation of foreign countries, which constitute the main scientific interest of the authors. The work was carried out taking into account the opinions of leading modern Russian scientists-specialists in the field of criminal law comparative studies. Analysis. The article studies the issues related to the definition in the criminal law of foreign countries of one of the basic concepts of criminal law doctrine – the concept of crime, as well as a number of emerging scientific problems. The complexity of the comparative legal study of this concept is aggravated by the absence of its legislative definition in a number of legal systems of modernity. To overcome this problem, the paper analyzes the concept of a criminal act characterized by signs of illegality (illegality), harmfulness (public danger), culpability and punishability. At the same time, the authors pay special attention to the issue of criminalization of criminal inaction. The historical and legal analysis of the attribute of causal relationship related to the act has been carried out. The article concludes with a study of doctrinal and legislative experience in the field of classification of crimes, as well as the practical significance of this issue for decision-making in the field of criminal legal assessment of committed acts. Results. The analysis allows the authors to formulate a number of reasonable and noteworthy conclusions of a comparative nature.