This document summarizes Lev Vygotsky's theory of cognitive development. It discusses five of Vygotsky's main assumptions: (1) complex mental processes begin socially and are gradually internalized; (2) thought and language develop independently before age 2 then become interdependent; (3) adults convey cultural ways of understanding the world to children; (4) children can perform more challenging tasks with assistance; and (5) challenging tasks promote maximum growth. The document also discusses contemporary applications of Vygotsky's ideas like scaffolding, guided participation, apprenticeships, and peer interaction.