This book explores a variety of central issues in the framework of Head-Driven Phrase Structure Grammar (HPSG), a major theory of syntactic representation that is becoming increasingly dominant, particularly in the domain of natural language computation. According to this theory, certain structurally key words ("heads") in any human language determine both the syntactic form and the semantic interpretation of the sentences they appear in. The separate chapters consider problematic phenomena in German, Japanese and English and suggest important extensions of, and revisions to, the current picture of HPSG.