Development of the category of "activity" in Soviet psychology is traced in the article. In is shown that originally, in L.S. Vygotsky's school the notion of activity existed under the disguise of "practical intellect" and was closely related to the idea of "mediation" of mental processes. Links between the studies of the "Kharkov group of psychologists" (A.N. Leontiev, P.Ya. Galperin, A.V. Zaporozhets, L.I. Bozhovich, P.I. Zinchenko) and L.S. Vygotsky's ideas are described.