To work out a residential treatment programme for children with persistent behaviour disorders, the literature on object constancy is reviewed. This term is used in different ways by the different authors. After summarizing the contributions of Hartmann, A. Freud, Spitz and the cognitive approaches of Fraiberg and S. Bell, the author describes in more detail the long way the child has to go in order to develop object relations with the characteristics of object constancy in the sense of Mahler and McDevitt. The distinction is made between the object constancy as a capacity of the ego to form object representations of a certain level and object constancy as a sub-phase of the separation individuation process. Of all the definitions of object constancy given by the different authors, the definition of Mahler and McDevitt is preferred. After an extreme long period of dependency on the physically present mother, the child can now function in her absence and have positive relations with other adults and children because of the sense of separateness of the self from the object.