Jean Piaget was a Swiss psychologist who developed a theory of cognitive development in children. He observed that children progress through four distinct stages of cognitive development as they age - sensorimotor, preoperational, concrete operational, and formal operational. At each stage, children develop new cognitive abilities and ways of thinking. Piaget's theory of cognitive development had a significant impact on the fields of education and child psychology and he is considered one of the most influential psychologists of the 20th century.