The history of word class research is characterized by two extreme positions. Up to the 19th century, it was believed that word classes were invariably of the Latin or Greek type and universal. In contrast to that, in the 20th century, the view prevailed that every language had its own specific and unique word class system. In the last decades, however, it has become apparent that despite the large number of word classes and word class systems there are typological restrictions with regard to the conceptualization of semantic features and morphosyntactic structures. This text approaches word classes and their categorial manifestations from the perspective of typology and language universals research. The authors in this volume discuss word class categorization in general (Part I), as well as word classes and word class systems of individual languages (Part II) from a typological-universal viewpoint and from diachronic and cross-linguistic perspectives.