Jean Piaget was born in Neuchâtel, Switzerland, on August 9, 1896. Many psychologists consider him to be the most influential developmental psychologist of the twentieth century. He made detailed observations of children's activities, talked to children, listened to them talking to each other, and devised and presented many tests of children's thinking.

It was Piaget who founded genetic epistemology, the study of the development of knowledge. Originally based on the observations he made of his own children, he concluded that younger children's intelligence is both qualitatively and quantitatively different to that of older children's. Piaget suspected that the way that we are able to form and deal with concepts changes as we move from childhood to adolescence.

Piaget's theory of cognitive development focuses on the organisation of intelligence and how it changes as children grow, and he identified a number of distinct stages of intellectual development (sensorimotor, pre-operational, concrete...