Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Theory of Cognitive Development, is a comprehensive theory about the nature and development of human intelligence first developed by Jean Piaget. It is primarily known as a developmental stage theory, but in fact, it deals with the nature of knowledge itself and how humans come gradually to acquire it, construct it, and use it. Moreover; Piaget claims the idea that cognitive development is at the centre of human organism and language is contingent on cognitive development. Below, there is first a short description of Piaget''s views about the nature of intelligence and then a description of the stages through which it develops until maturity.