Jean Piaget (1896-1980) was a pioneering psychologist who studied cognitive development in children, introducing the concept of genetic epistemology, which examines the origins of thinking. He proposed that children progress through discrete stages of development, which are marked by significant qualitative changes in their cognitive abilities, rather than a gradual increase in complexity. Key concepts in his theory include schemas, assimilation, accommodation, and equilibrium, with stages ranging from sensorimotor to formal operational that describe how children's thinking evolves.