This third edition of An Introduction to Functional Grammar has been extensively revised. While retaining the organization and coverage of the earlier editions, it incorporates a considerable amount of new material. This includes strengthening the grammar through the use of data from a large-scale corpus, upgrading the description throughout, and giving greater emphasis to the systemic perspective, in which grammaticalization is understood in the context of an overall model of language. The approach taken in the book overcomes the distinction between theoretical and applied linguistics. The description of grammar is grounded in a comprehensive theory, but it is a theory which evolves in the process of being applied.