Jean Piaget was a Swiss psychologist and philosopher who is best known for his work in the field of developmental psychology. He is credited with developing the theory of cognitive development, which states that children's mental development is based on their experiences and interactions with the environment. Piaget's research focused on how children acquire knowledge and understanding of the world around them, and how they use this knowledge to make sense of their environment. His work has had a lasting impact on the field of psychology and education.