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Transformational grammar (TG) or transformational-generative grammar (TGG) is, in the study of linguistics, part of the theory of generative grammar, especially of naturally evolved languages, that considers grammar to be a system of rules that generate exactly those combinations of words which form grammatical sentences in a given language. TG involves the use of defined operations called transformations to produce new sentences from existing ones. The concept was originated by Noam Chomsky, and much current research in transformational grammar has been inspired by Chomsky's Minimalist Program.

Noam Chomsky’s 1965 book Aspects of the Theory of Syntax developed the idea that each sentence in a language has two levels of representation — a deep structure and a surface structure. The deep structure represents the core semantic relations of a sentence, and is mapped onto the surface structure (which follows the phonological form of the sentence very closely) via transformations. Chomsky believed there are considerable similarities between languages' deep structures and that these reveal properties, common to all languages, that surface structures conceal. However, this may not have been the central motivation for introducing deep structure; transformations had been proposed prior to the development of deep structure as a means of increasing the mathematical and descriptive power of context-free grammars. Similarly, deep structure was devised largely for technical reasons relating to early semantic theory. Chomsky emphasizes the importance of modern formal mathematical devices in the development of grammatical theory:

But the fundamental reason for [the] inadequacy of traditional grammars is a more technical one. Although it was well understood that linguistic processes are in some sense "creative," the technical devices for expressing a system of recursive processes were simply not available until much more recently. In fact, a real understanding of how a language can (in Humboldt's words) "make infinite use of finite means" has developed only within the last thirty years, in the course of studies in the foundations of mathematics.

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