The project investigates the synchronic and short-term diachronic syntax of Datives (DAT) in Pol[ish] as compared to Ice[landic] and Ru[ssian], and its implications for general linguistic theory. DAT Case is a particularly revealing phenomenon at the interface of morphology, semantics, and syntax. It occurs on (i) indirect objects; (ii) facultative arguments of (in)transitives ('free datives'); (iii) experiencers of psychological/physical state predicates, modals, derived (REFL[exive]) decausatives and impersonals; (iv) secondary predicates in (Ru and Pol) embedded infinitives; (v) Main Clause Infinitives (MCI) in Ru and, marginally, related constructions in Pol. Among these, (iii) involves partly lexical, partly structural factors, and relates to issues of argument-structural variation and diathesis. Moreover, (iii)-(v) show differences in grammaticality between closely related languages (Pol and Ru), but surprising commonalities between genetically unrelated languages (Pol and Ice). Strikingly, the relevant subtypes of middles/impersonals in Pol evolved within a short time frame (< 200 years); Ice is currently undergoing a similar development (Maling & Sigurjónsdóttir 2002). While previous treatments focus on the conceptual semantic core of DATs, the facts of cross-linguistic variation and change under (i)-(v) call for an explicit, parametric syntactic analysis. We will analyze the given phenomena and their empirical variation through experimental studies of linguistic acceptability and corpus-linguistic methods, taking into account, among others, measures of productivity. Then we will provide a syntactic analysis of DAT licensing. Finally, we will test the predictions of this theory against the facts of short-term diachronic change. The research is facilitated by recent advances in Case Theory - Nanosyntax (Jabłońska 2007; Caha 2009) – one of the most dynamic areas of current Slavic morphosyntax research, as well as by work on the nature of syntactic object preposing (Bailyn 2003, Witkoś 2008). The project crucially relies on linking up the competences of the Pol team (syntactic theory, conditions on movement, case licensing and argument structure [AS]) with those of the German team (subjects and AS, diachronic syntax, corpus linguistics, Pol vs. Ru and Ice).