Comparison across formal languages is an essential part of formal linguistics. The study of closely-related varieties has proven extremely useful in comparing differences that might otherwise appear unrelated, and has helped to identify the core principles of Universal Grammar. This comprehensive handbook serves two functions. It will provide a general and theoretical introduction to comparative syntax, its methodology, and its relation to other domains of linguistic inquiry; and it will provide a systematic selection of the best comparative work being done today on those language groups and families where substantial progress has been achieved. With top-notch editors and contributors from around the world, this volume will be an essential resource for scholars and students in formal linguistics.