Generative semantics is the name of a research program within linguistics, initiated by the work of various early students of Noam Chomsky: John R. Ross, Paul Postal, and later James McCawley. George Lakoff was also instrumental in developing and advocating the theory.
The approach developed out of transformational generative grammar in the mid-1960s, but stood largely apart from, and in opposition to, work by Noam Chomsky and his later students. This move led to a more abstract framework and lately to the abandonment of the notion of deep structure.
A number of ideas from later work in generative semantics have been incorporated into cognitive linguistics, Head-Driven Phrase Structure Grammar (HPSG), Construction Grammar, and into mainstream Chomskyan linguistics.
The nature and genesis of the program are a matter of some controversy and have been extensively debated. Generative semanticists took Chomsky's concept of Deep Structure and ran with it, assuming (contrary to later work by Chomsky and Ray Jackendoff) that deep structures were the sole input to semantic interpretation. This assumption, combined with a tendency to consider a wider range of empirical evidence than Chomskyan linguists, led generative semanticists to develop considerably more abstract and complex theories of deep structure than those advocated by Chomsky and his students—and indeed to abandon altogether the notion of "deep structure" as a locus of lexical insertion.
Throughout the late 1960s and 1970s, there were heated debates between generative semanticists and more orthodox Chomskyans. The generative semanticists lost the debate, insofar as their research program ground to a halt by the 1980s. However, this was in part because the interests of key generative semanticists such as George Lakoff had gradually shifted away from the narrow study of syntax and semantics.
“Interpretive” vs. “generative” semantics 
The controversy surrounding generative semantics stemmed in part from the competition between two fundamentally different approaches to semantics within transformational generative syntax. The first semantic theories designed to be compatible with transformational syntax were interpretive. Syntactic rules enumerated a set of well-formed sentences paired with syntactic structures, each of which was assigned an interpretation by the rules of a separate semantic theory. This left syntax relatively (though by no means entirely) "autonomous" with respect to semantics, and was the approach preferred by Chomsky.
In contrast, generative semanticists argued that interpretations were generated directly by the grammar as deep structures, and were subsequently transformed into recognizable sentences by transformations. This approach necessitated more complex underlying structures than those proposed by Chomsky, and more complex transformations as a consequence. Despite this additional complexity, the approach was appealing in several respects. First, it offered a powerful mechanism for explaining synonymity. In his initial work in generative syntax, Chomsky motivated transformations using active/passive pairs such as "I hit John" and "John was hit by me", which despite their identical meanings have quite different surface forms. Generative semanticists wanted to account for all cases of synonymity in a similar fashion—an impressively ambitious goal before the advent of more sophisticated interpretive theories in the 1970s. Second, the theory had a pleasingly intuitive structure: the form of a sentence was quite literally derived from its meaning via transformations. To some, interpretive semantics seemed rather "clunky" and ad-hoc in comparison. This was especially so before the development of trace theory.
^ There is little agreement concerning the question of whose idea generative semantics was. All of the people mentioned here have been credited with its invention (often by each other).
^ Strictly speaking, it was not the fact that active/passive pairs are synonymous that motivated the passive transformation, but the fact that active and passive verb forms have the same selectional requirements. For example, the agent of the verb kick (i.e. the thing that's doing the kicking) must be animate whether it is the subject of the active verb (as in "John kicked the ball") or appears in a by phrase after the passive verb ("The ball was kicked by John").
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