The problems of determinants and mechanisms of child development are the most important and difficult problems in child psychology. In contemporary Western psychology, there are detailed studies of the development of perception, memory, cognition, and other mental processes in each period of the child's life. But, what is almost always missing is an explanation of why the child transits from one period of development to the next. Even such a giant of child psychology as Jean Piaget could not give a satisfactory answer to this question. His explanation of the reasons for the child's transitions from one stage to the next was that these transitions “become necessary with development” (Piaget, 1971, p. 9). The weakness of such an explanation was formulated by Piaget (1971) himself: “This solution is difficult to prove. It is even difficult to express or to explain” (p. 9). Dissatisfaction of developmental and child psychologists with the absence of a powerful theory of child development reveals itself in the advocation by some of them of reductionist approaches, in which, as an example, developmental biology is suggested as a “metatheory for cognitive development” (Bjorklund, 1997, p. 144).

An innovative approach to the problems of determinants and mechanisms of child development was suggested by Vygotsky (1978, 1986, 1997, 1998). Because of the translation of practically all of Vygotsky's works into English, as well as the availability of numerous reviews of his theory by Western psychologists, English-reading psychologists are fairly familiar with his ideas.