This chapter presents conceptual definitions of grammar, and describes some of the key approaches to the models and theories of grammar. It takes a historical look at prescriptions and descriptions of grammar, and charts the evolution of books about grammar. It illustrates how variation plays a central role in a discussion of grammar and considers the issues relating to teaching and learning grammar. It illustrates how the advent of corpora has allowed us to capture grammatical usage and monitor grammatical change, and finally discusses the challenges of describing the widely changing and varying grammar of English in its global contexts of use.