This book describes and evaluates alternative approaches within Systemic Functional Linguistics (SFL) to representing the structure of language at the level of form. It assumes no prior knowledge of SFL, and can therefore be read as an introduction to current issues within the theory. It will interest any linguist who takes a functional approach to understanding language. Part 1 summarizes the major developments in the forty years of SFL's history, including alternative approaches within Halliday's own writings and the emergence of the Cardiff Grammar as an alternative to the Sydney Grammar . It questions the theoretical status of the 'multiple structure' representations in Halliday's influential Introduction to Functional Grammar (1994), demonstrating that Halliday's model additionally needs an integrating syntax such as that described in Part 2. Part 2 specifies and discusses the set of 'categories' and 'relationships' that are needed in a theory of syntax for a modern, computer-implementable systemic functional grammar. The theoretical concepts are exemplified at every point, usually from English but occasionally from other languages. The book is both a critique of Halliday's current theory of syntax and the presentation of an alternative version of SFL that is equally systemic and equally functional.