The present paper proves the relevance of the diachronic approach to grammaticalization research. The article contains definitions of the concept of grammaticalization and makes a distinction between diachronic and historical grammar. The theoretical review is based on research carried out by such scholars as I.A. Baudouin de Courtenay, W. von Humboldt, E. Kuryłowicz, B. Heine, Ch. Lehmann, E. Traugott, J. Bybee and others. Grammaticalization is defined as a process whereby lexical units turn into grammatical units or the latest increase their grammatical status. It is noted that a diachronic approach to this process enables researchers to identify the grammaticalization sources, find out the patterns and possible ways of development of universal grammaticalization paths in different languages and prove the unidirectionality hypothesis. This approach also provides the explanatory value of a linguistic theory, allows us to identify the cognitive and communicative factors, analyze the dynamics of grammatical changes and find out the similarities between languages in the most explicit way. Based on the example of the English grammaticalized construction be going to, it is proved that the grammaticalization process is gradual and requires a diachronic approach for a more comprehensive analysis of intermediate stages of development of grammatical units.