Lev Vygotsky was born in Belarus in 1896, when it was still part of the USSR. Vygotsky had a law degree and had never studied psychology, but he began to study child development and wrote many papers on the topic. Vygotsky died in 1934, the same year that his book "Thought and Language" was published. Although his theories were not accepted by the Russian government, and therefore not widely known until the end of the Cold War, they have been very influential since their release.
Vygotsky believed that a complete model of child development must explain it as a single, comprehensive process. He divides childhood into several periods in which particular psychological processes are developing. His model also takes into account the whole child, rather than particular elements of development in isolation. He posits that children develop in certain areas in response to a conflict or contradiction between what a child is able to do and what he desires to do, or what the situation demands of him.
The zone of proximal development (ZPD) is explained by Vygotsky as the distance between a child's independent problem-solving ability and her ability to solve problems when guided by an adult or a more skilled peer. This difference between independent and supported problem solving indicates which skills a child is currently in the process of developing. This is related to the idea of "scaffolding," in which a teacher supports a student in completing a task that he cannot yet complete independently.
The ZPD is not fixed for a child at a certain age, but depends on the child's cultural and educational context. Teachers can use the idea of the ZPD to determine where a student operates currently, in terms of maturity and development. It can also be used to determine which psychological functions a student needs to develop to move on to the next level. Finally, Vygotsky's theory reinforces the need for guided practice in the classroom so that students can become independent with a new skill.