Introduction: The article is devoted to the analysis of the legal category of «internal affairs bodies» and related categories that specify the composition of state apparatus. Rationale is determined by the provisions of theoretical and doctrinal nature, which vaguely formulate the concept of «internal affairs bodies», based on the formal–legal criterion and substantive features of the law enforcement segment of the state apparatus. Methods: When analyzing theoretical provisions of the doctrine, which reveal the concepts under study, comparative analysis and method of historical normative acts analysis were used. Statutory documents of by-law nature that define the system of the Ministry of Internal Affairs were considered. Results: Formal and substantive (material) criteria defining the system of internal affairs bodies are identified. Formal criteria include fixing the system of internal affairs bodies in normative and administrative acts, departmental organization of internal affairs bodies’ management. Substantive criteria include law enforcement functions performed by internal affairs bodies; the presence of paramilitary units within them; specific features of the public service in the internal affairs agencies.The link between the indicated category and the implementation of the concept of powers separation (separation of the executive branch of power) is noted as well as uncertainty in the historical–legal retrospective sphere of the status of internal affairs bodies as subordinate to the Ministry of Internal Affairs; the dominance of the formal criterion of definition over the substantive one;uncertainty as to the status of some functionally related parts of the state apparatus outside the Russian Ministry of Internal Affairs system.