This handbook provides a critical guide to the most central proposition in modern linguistics: the notion, generally known as Universal Grammar, that a universal set of structural principles underlies the grammatical diversity of the world's languages. Part I considers the implications of Universal Grammar for philosophy of mind and philosophy of language, and examines the history of the theory. Part II focuses on linguistic theory, looking at topics such as explanatory adequacy and how phonology and semantics fit into Universal Grammar. Parts III and IV look respectively at the insights derived from UG-inspired research on language acquisition, and at comparative syntax and language typology, while part V considers the evidence for Universal Grammar in phenomena such as creoles, language pathology, and sign language. The book will be a vital reference for linguists, philosophers, and cognitive scientists.