Jean Piaget's theory of cognitive development outlines how children's intelligence evolves through four distinct stages: sensorimotor, preoperational, concrete operational, and formal operational. Piaget posited that cognitive development is influenced by biological maturation and environmental experiences, with processes like assimilation, accommodation, and equilibration being central to understanding new information. His work has significantly impacted educational practices, although it has faced criticism regarding research methods and the underestimation of children's abilities.