This paper asserts the "unquestionable" relevance of linguistic insights in the training of and subsequent use by teachers of English as a foreign language. Although the author agrees with Chomsky's view that linguistics has nothing to offer the teacher in the form of specific proposals for language teaching methodology, he argues that linguistics can give the teacher the understanding of the phonetic, morphological, syntactic, semantic and usage aspects of the language necessary for classroom effectiveness. Because of the dominance of transformational grammar in recent linguistic research, it is considered imperative that the TEFL teacher be aware of the terminology, tenets and results of this particular approach to language. The paper is, therefore, intended to provide the teacher with such an overview of transformational grammar and te direction which it is taking. In addition one specific approach designed to make the results of linguistic research directly relevant to the TEFL teacher is outlined: the development of a book intended to make the teacher aware of the sorts of errors most likely to occur among learners of English, the linguistic facts which underly these errors, and the way or ways in which the errors might be remedied. Work on such a book is presently being carried out at the Language Research Foundation in Cambridge, Massachusetts. (FWB)