The thrust of this book is to provide a model of lexical relations which 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. Aside from the conceptual simplification that results from this move, the book demonstrates that several empirical and theoretical benefits accrue to this architecture, in particular, many salient properties of morphological processes are shown to reduce to inherent, formal properties of the organization of the lexicon.