Jean Piaget was a Swiss psychologist known for his theory of cognitive development. He identified four stages of development: sensorimotor (birth to 2 years), preoperational (2 to 7 years), concrete operational (7 to 11 years), and formal operational (11 to 15 years). Piaget's theory focused on how children's thinking changes as they interact with the world, with concepts like assimilation, accommodation, and equilibration describing how children incorporate new information and move between stages of development.