The chapter turns to the period of the formation of Vygotsky's theory. In the last few years of his life, Vygotsky was not satisfied with the level of development of his own theory. He reformulated his own theory in order to respond to the theoretical and practical challenges that arose in the process of his development both as a scholar and practitioner. Vygotsky developed a set of new concepts such as a psychological system, meaning and sense, perezhivanie, the social situation of development, the crisis that enabled him to investigate consciousness and cultural development in a new light. During that period, serious tensions and disagreements appeared within Vygotsky's circle on important theoretical and methodological issues, first of all, about the relation between consciousness and activity.