This is the first of several chapters discussing the developmental tradition, with this chapter focusing on Piaget’s theory. It outlines the major positions of Piaget towards the origins of intelligence (a mechanism of biological adaptation) and its nature. That is, constructive process organizing interactions with the environment into structures of mental operations bridging the individual with the environment by enabling her to assimilate current encounters into existing structure and accommodate structures to the peculiarities of the current encounter, ensuring cognitive development. The major stages of cognitive development (sensorimotor, preoperational, concrete and formal thought) are outlined. The theory is evaluated from the point of research that scrutinized its basic postulates. It is concluded that Piaget’s work refreshed the study of the human mind throughout the twentieth century, introducing new themes and questions and a new approach to viewing development that needs to be preserved. However, many of its specific answers (structure of the whole, priority of logical reasoning, etc.) were not supported by research.