The article provides a theoretical overview of the history of the study of function words in Russian linguistics. Various approaches to function words are noted, due to their structural, semantic and functional diversity. Particular attention is paid to the issue of the polyfunctionality of function words, their ability to act as different parts of speech in different sentences. It is noted that the constant expansion, deepening, clari­fication of the theory of function words in Russian linguistics is due to the mobility of this class of linguistic units, the permeability of its boundaries and the polyfunctionality of function words. Each category of function words is continuously replenished at the expense of both units of other function parts of speech and significant words. New function and polyfunctional units are most often composite, include several words; in addition, they are typically used in syntactic constructions, in connection with which the grammar of constructions is a promising direction of linguistic research.