Piaget was a Swiss psychologist who studied cognitive development in children. Through observing his own children, he developed a stage theory of cognitive development consisting of 4 stages: sensorimotor (birth to age 2), preoperational (ages 2 to 7), concrete operational (ages 7 to 11), and formal operational (age 11 into adulthood). Key aspects of cognitive development according to Piaget include schemas, assimilation, accommodation, and equilibration as children adapt their understanding of the world.