The text of P.Ya. Galperin at the All-Union Conference on Psychology, held at the Academy of Pedagogical Sciences of the RSFSR in 1953. In this report, for the first time in a holistic form, the foundations of his world-famous method for the systematic, step-by-step formation of mental actions and concepts are presented. The author states that the question of mental actions, i.e. about what our ability to solve “in the mind” different tasks is part of the broader problem of the activity of human consciousness, taken not in philosophical or socio-historical, but in specifically its psychological content. The way of formation of mental actions that make up a significant part of what is taught in school is described as a successive transition from mastering actions with objects through action in terms of audible speech to transferring an action to the mental plane. The stages of this process are singled out: 1) drawing up a preliminary view of the assignment; 2) mastering the action with objects; 3) mastering the action in terms of audible speech; 4) transferring an action to the mental plane; 5) the final formation of mental action. A detailed description of each stage is given. The author emphasizes that the step-by-step formation of mental actions remains the actual content of the learning process of new skills, regardless of the individual or collective nature of instruction, and the scale of the main parameters retains value for evaluating the learned action regardless of the actual content and quality of this training. In conclusion, the author expresses the hope that the knowledge of the process of step-by-step formation of mental actions and their other parameters will serve as a theoretical basis for the management of full-fledged training.