Abstract: The subject of this research is the part-of-speech affiliation of modal words – a special group of lexemes that are irreplaceable, syncategorematic, and lack formal unity. The author examines four most widespread approaches towards determination of the part-of-speech status of modal words, modal semantics, in particular, and the function of modal meaning; provides summary of the existing in Russian linguistics classifications of modal words, and lists their basic lexical-semantic categories. In the course of studying the genesis of modal words, the author correlates them with other parts of speech. For determination of the part-of-speech status, emphasis is placed on the degree of manifestation within this group of lexemes of semantic, morphological, syntactic and word-forming characteristics, namely the compliance of modal words with the listed criteria. The novelty consists in the analysis of the part-of-speech affiliation of modal words through the prism of the traditional semantic-syntactic approach, considering the formal-morphological aspect of modal words, which allows accumulating the avaliable knowledge on modal words, as well as compiling holistic representation on the nature of their origin and functionality. The research materials can be used in teaching morphology of the modern Russian language, as well as in preparation of lectures and textbooks on cognate disciplines. The conclusion is made that in synchrony, modal words represent an isolated group that is in-between of autosemantic and functional words. The recommendation to classify this group as a separate part of speech remains polemical; however, segregation of modal words in the grammatical system is unarguable.