Most psychoanalysts in the twenties of the last century were of the opinion that young children are not subject to the analytical method of treatment. Melanie Klein clearly had doubts about this state of affairs; she believed that instead of turning these patients away, the method should have been changed. On this path, she made three brilliant discoveries. The first was the invention of the psychoanalytic technique of play: she saw in this spontaneous activity of any child the equivalent of dreams, and therefore a clear road and access to the unconscious of her little patients. Second: she was able to show that the Oedipus complex and the Super-Ego arise from the first stages of development of mental life, that is, long before the age at which Freud attributed their appearance. Third: she discovered that a child of three years and four months of age is capable of transference, which can be interpreted, which allows psychoanalytic treatment to be carried out. Several truly revolutionary articles that make up this book laid the foundation for child psychoanalysis.