This is a general introduction to grammaticalization, the change whereby lexical terms and constructions come in certain linguistic contexts to serve grammatical functions, and, once grammaticalized, continue to develop new grammatical functions. The authors synthesize work from several areas of linguistics. The second edition has been thoroughly revised with substantial updates on theoretical and methodological issues that have arisen in the decade since the first edition, and includes a significantly expanded bibliography. Particular attention is paid to recent debates over directionality in change and the role of grammaticalization in creolization.