Ordinary lexical items and constructions come in certain linguistic contexts to serve grammatical functions, and, once grammaticalized, continue to develop new grammatical functions. Thus nouns and verbs may change over time into grammatical elements such as case markers, sentence connectives, and auxiliaries. The processes whereby these changes occur are known as "grammaticalization." This book is the first general introduction to the subject. The authors synthesize work from several areas of linguistics, in an account of the typical semantic, morphosyntactic, and phonological changes that accompany grammaticalization. Data is drawn from many languages, including Ewe, Finnish, French, Hindi, Japanese, Malay, and especially English, and a wide range of linguistic phenomena is discussed, including clause combining, auxiliation, case marking, erosion of inflections, and renewal of categories. This account is unique in its wide-ranging discussion of the processes of grammaticalization. It will be a valuable and stimulating textbook not only for all linguists interested in the development and decay of grammatical forms, but also for readers in psychology and anthropology interested in deepening their understanding of the interaction of language structure and use.