Piaget's theory of cognitive development describes 4 stages that children progress through as they develop intellectually. The stages are: sensorimotor (birth to age 2) where children learn through senses and actions; preoperational (ages 2 to 7) where symbolic thought and language emerge but reasoning is still concrete; concrete operational (ages 7 to 11) where logical and systematic manipulation of symbols related to concrete objects develops; and formal operational (age 12 and up) where abstract reasoning and thinking about hypothetical situations and concepts emerges. Piaget's theory emphasizes that cognitive development progresses as children interact with the physical world and gradually develop new ways of thinking.