The paper presents P.Ya. Galperin’s ideas of the psychological content of interiorization. P.Ya. Galperin considered the psyche as subject’s orienting activity and defined its functional role in the subject’s human activity. The internal orienting activity retains structural similarity with practical activities. The internal plan of activity is deemed as a «reserve field» that is necessary for testing actions in certain situations and disclosing the conditions for implementing them. The mental image arises as a component of activity, represents the features of objects being necessary for orientation and construction of actions. P.Ya. Galperin distinguishes features of two types of situations: where the psyche is not the primary matter and where the psyche is the primary matter. The principal ideas of the gradual development of mental actions theory and the method of directed development of mental processes as a re-creation of the logic of mental phenomena that reveal the psychological content of interiorization are discussed. The author justifies the concept of «subject logic of developed actions» that objectively dictates the logic of organizing not only cognitive, but also mental processes.