Jean Piaget was a Swiss psychologist and biologist known for his work on child development. He developed a theory of cognitive development that described four stages that children progress through as they interact with their environment - sensorimotor, preoperational, concrete operational, and formal operational. His theory focused on how children's understanding of the world evolves through biological maturation and social experience from birth until adulthood. Piaget conducted extensive observations of children to develop his stages of cognitive development and theories about how children think and learn.