This short book is two books in one. First, it is a genuinely introductory introduction to the main concepts of Systemic Functional Grammar (SFG) for the 21st century. Drawing on recent research, it focusses on the structure of the clause in English, bringing out clearly (i) the 'multifunctional' nature of language, and (ii) the way in which structures are the result of 'choices between meanings'. It is an 'extension' of Halliday's SFG in several ways, including the introduction of new elements that are of growing importance in the language, and a 'simplification' of it in that it shows how the many 'strands of meaning' in a clause can be expressed in a single structure. But this is also a book for experienced linguists (who may include the teachers of the first group) who are interested in a scholarly work which (i) compares the two main current versions of Systemic Functional Grammar with respect to the structure of the English clause, and (ii) gives reasons for every decision to prefer one analysis to another. This 'book within a book' is achieved through a generous use of extended 'footnotes'. The Cardiff Grammar version of SFG is based as firmly in the core principles of SFG principles as the Sydney Grammar (the version in Halliday's Introduction to Functional Grammar). Halliday, however, describes the development in the Sydney Grammar since the 1970s as expansions into new areas beyond what he has aptly termed the 'lexicogrammar'. In contrast, the Cardiff Grammar's description of English lexicogrammar (and other languages) has made significant advances since the 1970s, under the influence of eight major factors. Versions of this book are being published concurrently in Chinese (by Peking University Press) and Spanish (University of Plata del Mar Press).