Lexicalization, a process of language change, has been conceived in a variety of ways. In this up-to-date survey, Laurel Brinton and Elizabeth Traugott examine the various ideas that have been presented. In light of contemporary work on grammaticalization, they then propose a new, unified model of lexicalization and grammaticalization. Their approach is illustrated with a variety of case studies from the history of English, including present participles, multi-word verbs, adverbs, and discourse markers, as well as some examples from other Indo-European languages.