This document summarizes Lev Vygotsky's constructivist theory of cognitive development. It explains that Vygotsky believed learning is influenced by social development and occurs in the zone of proximal development, which is the level of development attainable by children with guidance from peers or teachers. The document also notes how Vygotsky's theory fits well in classrooms by helping teachers assess students' abilities and determine their zone of proximal development to provide appropriate challenges and mentorship.