Focusing on the relation between functional categories and lexical and phrasal categories in Arabic dialects, Benmamoun proposes that universally functional categories are specified for categorial features which determine their relation with lexical categories. Language variation is attributed to differences with respect to the categorial feature specifications of functional categories and how they interact with lexical categories. The book brings new insights to issues related to the syntax of functional categories, the relation between syntax and the morpho-phonological component, and comparative syntax.