G. S. Kostiuk made a significant contribution to the study of the problem of determination of activity of personality and of the role of a subject's activity in his psychological development. Activity was studied by Kostiuk in the unity of its meaningful, operational, and motivational aspects, as a system of processes related to solution of cognitive and practical tasks. He understood psychological development as a qualitative and structural improvement of mental activity forced forward by internal contradictions. When stressing that development of an individual is to a large extent determined by his social environment, Kostiuk indicated that in the long run it is determined by the content and manner of his activity in this environment. Educational efforts devoted to promotion of such activity and of multifarious relations between the individual and his environment must contribute also to resolution of inner conflicts within his personality and at the same time to provoke new ones necessary for its further development.