The author supports the opinion that L.S. Vygotsky's theory contains two different interpretations of interiorization - as transformation of 1) outer individual forms of behavior into inner individual ones, 2) collective forms of behavior into individual ones. The paper describes subtypes of each of the two interiorization types, contents and mechanisms of interiorization, as well as analyzing evolution of L.S. Vygosky's views on interiorization in the context of the development of his scientific thought.