In this pioneering study, a world-renowned generative syntactician explores the impact of phenomena known as 'third factors' on syntactic change. Generative syntax has in recent times incorporated third factors - factors not specific to the language faculty - into its framework, including minimal search, labelling, determinacy and economy. Van Gelderen's study applies these principles to language change, arguing that change is a cyclical process, and that third factor principles must combine with linguistic information to fully account for the cyclical development of 'optimal' language structures. Third Factor Principles also account for language variation around that-trace phenomena, CP-deletion, and the presence of expletives and Verb-second. By linking insights from recent theoretical advances in generative syntax to phenomena from language variation and change, this book provides a unique perspective, making it essential reading for academic researchers and students in syntactic theory and historical linguistics.