This is the introductory chapter of this book, which primarily focuses on the theory of Lexical Functional Grammar (LFG) as it has developed since its inception in the late 1970s. The book examines the two syntactic structures of LFG, the constituent structure and the functional structure. It discusses the nature of the linguistic information they represent, the formal structures used to represent them, and the relation between the two structures. The book outlines the formal architecture of LFG. It provides the concepts of the theory more explicitly by presenting a series of sketches of the syntax and semantics of a range of representative linguistic phenomena.