This dissertation examines the interplay of Situation Aspect and Viewpoint Aspect in two classes of constructions in Polish: i) anticausatives with and without Dative subjects, ex. (Jankowi) złamały się okulary John broke the glasses involuntarily / The glasses broke; and ii) Factual Imperfectives, ex. Jadłam obiad I have eaten my dinner. With the first type of construction, the concern is why the unintentional causer reading is obtainable only in telic contexts and how two classes of Polish anticausatives – one with prefixed verbs and one with unprefixed verbs – differ structurally. The question that arises with analyses of Factual Imperfective constructions (FIs) is about the untypical role of imperfective aspect: how is it possible that the imperfective morphemes that the Slavic literature (almost) unanimously pairs with English Progressives do not neutralize telicity as the Progressive does? The dissertation finds evidence supporting the view that Slavic lexical prefixes are telicity markers and secondary predicates (cf. Svenonius 2004, Žaucer 2009). It argues that imperfectives that lack any aspectual morpheme and have the accusative case assigned to a direct object, nonetheless possess a non-overt aspectual operator located in the domain of Viewpoint Aspect. The dissertation enriches the recent discussions concerned with the puzzle of how to account for various readings of the imperfective (Cipria & Roberts 2000, Hacquard 2006, Deo 2009) by providing a semantic analysis of an interpretation not attested in languages like French, Italian and Spanish on which the discussions have focused so far. I argue that the Russian and Polish patterns dubbed as Existential Factual Imperfectives (cf. Grønn 2003) carry a silent Epistemic Modal that selects the imperfective due to its right semantic type: <s,t> in contrast to the <t> type of the perfective. From a cross-linguistic perspective, this study is relevant for syntactic and semantic theories of aspect in natural language especially for the theory of imperfectivity. From a language specific perspective, this study aims to provide a deeper understanding of the particular aspect-related conditions that play a role in licensing oblique (Dative) causers in Polish.