The thrust of this book is to provide a model of lexical relations that reconciles the lexicon's idiosyncratic and productive aspects. Building on work in Head-driven Phrase-Structure Grammar, an organization of lexical knowledge is proposed called the Type Underspecified Hierarchical Lexicon through which partial regularities, medium-size generalization, and truly productive processes receive a unified model. Its basic thesis is that all lexical relations reduce to categorization (the membership of the two related lexemes in a common category) and that category intersection is the only mechanism needed to model lexical processes, provided lexical items can be stored partially underspecified as to their category membership.