The first part of the book begins with a reflection on the goals of modern linguistic theory, and investigates the principles of human language, in an effort to subsume the regularities of particular grammars under a small set of morphosyntactic and semantic primitives. The second part examines the clausal structure - both the CP-layer and the IP-layer - from a comparative perspective, which directly relates to the fundamental questions of universality, linguistic variation, and learnability addressed in the first part of the book. With chapters written by world-leading linguists who analyze a wide range of old and new phenomena, the volume will be a valuable resource for researchers and students interested in theoretical linguistics and language development.