This document summarizes Piaget's four stages of cognitive development: 1) The sensorimotor stage involves physical and sensory experiences of infants from birth to age 2. 2) The preoperational stage is from ages 2 to 7 where children use language and symbols for thinking but are egocentric. 3) The concrete operational stage from ages 7 to 11 involves logical and systematic manipulation of symbols related to concrete objects. 4) The formal operational stage from age 11 onward involves abstract reasoning and thinking about hypothetical concepts.