The document summarizes Jean Piaget's theory of cognitive development. It describes Piaget's four stages of development: sensorimotor stage (birth to age 2), preoperational stage (ages 2 to 7), concrete operational stage (ages 7 to 11), and formal operational stage (ages 11 and up). It provides examples of cognitive skills developed at each stage, such as object permanence, symbolic thought, conservation, and hypothetical-deductive reasoning. The document also discusses Piaget's concepts of schemas, assimilation, and accommodation in cognitive development.