The paper is concerned with the origin and the development of the Accusative with Infinitive (AcI) construction in Slavic. Looking into the areal-typological, diachronic, and socio-typological parameters of the AcI construction, this paper introduces new Slavic dialectal and comparative material and reconstructs the developmental cline of this construction along two parallel pathways of grammaticalization of the second accusative complement in Proto-Indo-European. The grammaticalization of infinitival complementation, typical primarily of those Slavic varieties which acquired secondary analytical features, is distinguished from the grammaticalization of participial complementation which is commonly attested in the history of low-contact Slavic languages and dialects like Southwest Ukrainian and some Polish dialects. Special emphasis is placed on the interaction between infinitival and participial grammaticalization in the history of Slavic standard and non-standard varieties, which allows the author to substantiate an initial switching between the two pathways as attested in Old Church Slavonic and early standard varieties of (West) Slavic.