This paper describes the basic architectural concepts that underlie the formal theory of Lexical-Functional Grammar. The LFG formalism, which has evolved from previous computational, linguistic, and psycholinguistic research, provides a simple set of devices for describing the common properties of all human languages and the particular properties of individual languages. It postulates two levels of syntactic representation for a sentence, a consituent structure and a functional structure. These are related by a piece-wise correspondence that permits the properties of the abstract functional structure to be defined in terms of configurations of constituent structure phrases. The basic architecture crucially separates the three notions of structure, structural description, and structural correspondence. This paper also outlines some recent extensions to the original LFG theory that enhance its ability to express certain kinds of linguistic generalizations while remaining compatible with the underlying architecture. These include formal variations in the elementary linguistic structures, in descriptive notation, and in the arrangement of correspondences.