Jean Piaget was a Swiss psychologist who studied cognitive development in children. He observed his own children and others to develop a theory of four stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational. In each stage, children build cognitive structures through processes of assimilation and accommodation. Piaget's theory emphasizes that children are active learners who construct understanding of the world through experiences. His work influenced theories of education to focus on discovery learning tailored to children's developmental levels.