For the linguistic expression of the concept of knowledge, the Slavic languages use verbs deriving from the Indo-European roots *ĝnō and *u̯ ei̯d. They differ in terms of the availability of both types of verbs in the contemporary standard languages and in terms of their semantic range. As will be shown in this paper, these differences are interesting not only from a language-specific lexicological point of view, but also in the context of the intersection of lexicon and grammar. Covering the domain of ‘knowing how,’ the *ĝnō-based verb in Slovene (znati) has been extending into the domain of possibility and, on this basis, developing into a modal verb. While this development is not surprising from a typological point of view, it is remarkable from a Slavic perspective, since this particular grammaticalisation path towards possibility is otherwise unknown to Slavic. This peculiar feature of Slovene, which most probably relates to its long-lasting and intensive contact with German, is illustrated in the present paper by comparing Slovene to Russian on the basis of three main questions: 1) the semantic range of vedeti / vedat' and znati / znat', 2) the lexicalisation of ‘know how,’ and 3) the relation between knowledge, ability, and possibility. The focus is on contemporary Slovene and Russian, leaving a detailed diachronic investigation and the further embedding into a larger Slavic and areal perspective for future analyses.