Jean Piaget was a Swiss psychologist best known for his work in child development. He believed that children learn through a series of stages, each characterized by different ways of thinking and understanding the world. His theories emphasized that children actively construct their knowledge rather than just absorbing information from adults. Piaget identified four main stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational. Each stage represents a new level of understanding, from basic sensory experiences to complex abstract reasoning. His research has had a lasting impact on education and psychology, influencing how we understand children's learning processes.