This book explores a wide variety of theoretically central issues in the framework of Head-Driven Phrase Structure Grammar (HPSG), a major theory of syntactic representation which is becoming increasingly dominant, particularly in the domain of natural language computation. HPSG is a strongly lexicon-driven theory, like several others on the current scene, but unlike the others it also relies heavily on an explicit assignment of linguistic objects to membership in a hierarchically organized network of types, where constraints associated with any given type are inherited by all of its subtypes. This theoretical architecture allows HPSG considerable flexibility within the confines of a highly restrictive, mathematically explicit formalism, requiring no derivational machinery and invoking only a single level of syntactic representation. The separate chapters consider a variety of problematic phenomena in German, Japanese and English and suggest important extensions of, and revisions to, the current picture of HPSG.