This collection of papers opens a debate on the relationship between processes of grammaticalisation and systems of word-classes. It centers on the question of how diachronic grammaticalization processes and synchronic hierarchies of grammaticality relate to the lexico-grammatical categorization of the word inventory in natural languages (= systems of word-classes). Whereas systems of word-classes are normally only seen as an external system of reference for processes of grammaticalization, the papers in this volume inquire into the grammaticalization of the word-classes themselves (in the language system and in language acquisition).