The Evolution of Slavic Aspect examines the development of the Slavic aspectual systems in the historical era. The investigation focuses on developments that helped create the current east-west aspectual division of the Slavic languages. It documents not only how a group of eastern languages (East Slavic, Bulgarian and to a lesser extent Polish) have changed their systems through innovations in aspectual morphology and usage, but also why a group of western languages (Czech, Slovak, Sorbian, Slovene and Bosnian/Croatian/Serbian) have largely failed to do so. The division is the result of several phenomena, including: different fates of the Common Slavic tense system; differing productive models of perfectivizing prefixation in the individual languages; differing roles played by the reflexes of the Common Slavic nasal suffix -n?- in the derivation of perfective verbs; varying systems of nominal definiteness and their likely effect on the expression of verbal determination; finally, varying levels of German and Romance language contact in the western and eastern languages. It develops a Cognitive Grammar approach to the relevant historical processes and the grammaticalization of the category, including the issue of language contact.