Comparison across formal languages is an essential part of formal linguistics. The study of closely-related varieties has proven extremely useful in illuminating relations between cross-linguistic syntactic differences that might otherwise appear unrelated, and has helped to identify the core principles of Universal Grammar. Comparative studies have grown to the point where a reference work is needed to comprehensively explain the state of the field and makes its results more widely known, and this handbook fulfills that need. Its twenty-one commissioned chapters serve two functions: they provide a general and theoretical introduction to comparative syntax, its methodology, and its relation to other domains on linguistic inquiry; and they also 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.