The paper is dedicated to the 115th birthday of P.Ya.Galperin, the classic of national psychology (1902-2017). The purpose of the paper is to showP.Ya.Galperin’s contribution to fundamental and practiceoriented science, and alsoto revealthe link between these two approaches sides in his scientific heritage. The gist of his innovative psychological theory is disclosed. The key methodological principlesare highlighted: understanding of the scope and method of psychological science; psychophysical issue, i.e. the nature of psyche and the mental world; a psychophysiological problem, the issue of the relationship between psychology and physiology; the issue of human instincts in connection with historical social nature of consciousness; the notion of linguistic consciousness and the relation of language and thought; the principle of unity of history and theory in scientific knowledge. Terminological work carried out by P. Ya.Galperin is analyzed. The introduction of new concepts enriches and makes the conceptual apparatus of psychological sciencemore precise. The necessity to reveal each methodological provision to solve practical problemsis mentioned. The unity of theoretical and practical aspects in psychological theory developed by P. Ya. Galperin, its authentic nature and relation to the culturalhistorical psychology of L.S.Vygotsky, activity concepts of S.L. Rubinstein and A.N. Leontiev are shown. There are several directions in which the method of gradual development of mental functions and concepts organizes the process of teaching and learning, the mechanisms of perceptionand simultaneous mental processes, the phenomena of Jean Piaget are explained. To sum up, the inexhaustible possibilities of the practical use of the theory of P.Ya. Galperin in all areas of individual experience.