A new theory of personality development in ontogenesis is suggested. Proceeding from the unity, though not identity, of notions "individual" and "personality" the author refuses to consider "psychological development of an individual" and "development of an individual as a personality" as concepts fully coincident in their psychological characteristics. Development of a personality is determined not by the monopoly of a so-called "leading" activity specific for this or that age but by the activity-mediated form of interactions of the individual with his reference group. Basing on the theory of the activity mediation of inter-personal relations and of the personalization of individuals the author assumes that the personality developes within groups occupying hierarchical positions on the ontogenesis scale, while the type of the personality depends on the type and level of development of the group into which the personality is integrated. In any age period the personality passes through three microstages of development: adaptation, individualization, and integration. Three epochs of ascension of an individual to the state of social maturity-childhood, adolescence, youth-represent in their turn three macrophases of the personality development: adaptation, individualization, integration.