Piaget's theory of cognitive development proposes that children progress through four stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational. At each stage, children interact with their environment to construct an understanding of the world. The processes of assimilation, accommodation, and adaptation allow children to incorporate new experiences into existing mental structures and modify those structures based on new information. Piaget's theory emphasizes active learning through discovery and exploration appropriate to a child's developmental level.