Lev Vygotsky (1896-1934) was a Soviet psychologist whose short life produced one of the longest afterlives in educational thought. He wrote on development, language, mediation, and culture in ways that later transformed psychology, pedagogy, and, by extension, language teaching.
Vygotsky's influence on ELT is largely retrospective: he did not write for language teachers, but whole traditions of language pedagogy later organized themselves around questions he had already posed more sharply than most. Few thinkers have been posthumously put to work so productively.
Crucially, the project was Marxist by design. Vygotsky set out to rebuild psychology on the method of dialectical materialism, treating the higher mental functions as products of socially organised, tool-mediated activity with a real history rather than as fixed traits of the individual. He had no patience for the lazy version of this, the validating of claims by citing scripture, and wanted Marx's method rather than a garnish of Marx's quotations. That commitment is muted in much of the sociocultural tradition that language teaching inherited, which reached the West through abridged and editor-assembled texts during the Cold War. How much of his radicalism survived that passage, and how much was planed off, is the subject of a live and pointed debate (see The Americanisation of Vygotsky and the critique advanced by Anna Stetsenko).