This collection of selected papers addresses theoretical and empirical issues related to lexical categories, categorization and category change. Any grammatical description makes use of parts-of-speech. The proper set of lexical categories and the definitions of their properties cross-linguistically has been a remnant issue in linguistics since the beginnings of grammatical description. Besides, the traditional classification of lexical classes with their morphological, syntactic and/or interpretational properties has led to the emergence of mixed categories, which are problematic in linguistic theory, since the current systems, either feature-based or syntactic, have no means to express fuzziness. This volume addresses both these issues in two thematic parts. The first part, Categories and categorization, consists of papers that tackle the problem of defining categories and mixed-categories and its reflex on the inventory. The second part, Issues in category change, comprises investigations on category change, focusing on nominalizations, which is the test ground for a theory of category change and word formation. The papers included in this part discuss, among others, the similarities and mismatches between derived nominals and the corresponding verbs in terms of argument realization and eventive interpretation. The languages investigated in the volume include English, French, German, Greek, Japanese, Norwegian, Romanian, Russian, Spanish, and Swedish. This book targets researchers and advanced students in theoretical linguistics.