This paper describes the psychology of Lev S. Vygotsky, a Russian psychologist who lived and worked in the 20th century. It briefly describes his life and work and the impact of the October Revolution of 1917 on his career. The revolution guided the path of his career, bringing enormous changes in every aspect of Russian social life, including academic life. Western psychologists became interested in Vygotsky in the 1960s after translating his Thinking and Speech. The paper focuses on his sociocultural theory of cognitive development, as well as on the zone of proximal development, private speech, learning, education and development of mental functions. At the end of the paper, an effort was made to evaluate his theories as well as its contribution. Vygotsky was a leader in forming a theoretical approach that emphasizes the contributions of social and cultural context to intellectual development. The sociocultural approach to cognitive development, based on his ideas, proposes the idea that mental development can best be understood as a result of social and cultural experience, mostly social interactions. Children gradually learn to function cognitively as individuals in their culture through assistance, that is, through help that comes from the social environment, Gauvain (2019) emphasizes. Because of his interest in the social origins of cognitive functioning, Vygotsky was less concerned with children’s individual cognitive abilities than with a child’s potential for cognitive development through social experience. In order to assess and understand how cognitive development occurs, Vygotsky proposed the idea of a zone of proximal development, an area of sensitivity to learning (Gauvain, 2019). According to Berk (2015), Vygotsky believed that a child's learning takes place within the zone of proximal development (areas of approximate development) - those tasks that are too difficult for the child to solve on their own, can be solved with the help of adults and more competent peers.