He started to be interested in the development of thinking. There was little work done in the area, so he labeled it genetic epistemology. He provided support for the idea that children think differently than adults. Best known for identifying stages of mental development (Schema), and established fields of cognitive theory and developmental psychology.

He created a theory that explained the stages children pass through, which identifies 4 stages:

1. the sensorimotor stage

2. the preoperational stage

3. the concrete operational stage

4. the formal operation

He wrote about 70 books and more than 100 articles about human psychology.