Nearly 50 % of unsuccessful schoolchildren are mentally retarded. A differentiation into two groups uniform for the clinical-psychological and psychological-pedagogical description of mental retardation is suggested. The first group includes children with unformed processes of the spontaneous activity forms. The second group includes children with insufficiency of perceptive memorizing processes as the dominant factor in the structure of mental deviations. A set of three methods tested on the junior schoolchildren representative samplings is suggested.