Jean Piaget’s theme of investigation is mental development. The scientist paid special attention to the peculiarities of the process, which are proper for children. His scientific achievements are highly appreciated by the leading authorities in the field. The researcher introduced the modern concept of the mental growth of the infants. Experts all over the World still use this method nowadays. The reason is that such an approach is the most overall and comprehensive among the list of the existing ones. Piaget provided his own explanation of cognition and the way it progresses in the first years of life. What is more, he distinguished a range of moments to be passed in the action of acquiring knowledge and experience. The beginning period is, of course, started since the very moment a person is born. In other words, it is nominated as the sensorimotor level. It all remains to the closing of the operational stage when the young reaches his/her adulthood. Piaget described child’s acquaintance with everything that surrounds him or her. Broadly speaking, there are six steps. The first one lasts for a month when a newborn develops reflexes and is not yet able to respond to the things happening around. The next three months infants acquire habits and coordinate their sensation. Then till the age of eight months, a baby wants to get to know more about different objects, for example, a rattle. The fourth stage is realizing the existence of various things whether they see them or not. They can think over and do something intentionally now. Later toddlers already know the new possibilities and try to reach interesting results with the help of the objects they have previously touched. The following phases are two, seven, eleven and sixteen years old. The research paper could not be written at once. The theory was made up step-be-step as Piaget examined the behavior of his own offsprings. Having three kids, he thoroughly observed their behavior. Piaget was intrigued by the roots of children’s errors. He used a descriptive method and regularly made notes of the phenomena he detected. As the descendants became older, he conducted experiments. Additionally, the expert used the practice of Barbel Inhelder, a Swiss psychologist, who made a great discovery on the stage of human up-growth when a child transforms into a teenager. Jean Piaget’s investigation is a real breakthrough in the field of psychology. It clears up many points that seemed inexplicit and impenetrable before. The distinguished scholar presented a certainly new viewpoint on how kids create their model of society and their own living. These ideas are not only theoretical material but also the information to be used practically. Moreover, it is a solid base for further discoveries. People from different countries, who are really into psychology, can have easy access to this priceless work. It is written very intelligibly with a lot of examples so that every ordinary person can understand.