Multifamily therapy (MFT), the treatment of several families simultaneously, is an established method of psychotherapy used in a wide range of diagnoses. However, there is limited knowledge about the exact factors of action of this treatment method. The development of MFT as a separate form of therapy is presented, as are its previous applications in the treatment of patients with eating disorders. Known factors of change are critically evaluated, new factors of change are discussed and the practical application in MFT in the context of inpatient treatment of adolescents with eating disorders is demonstrated.Processes in the large group that have not yet been considered, such as emotional contagion, exaltation of emotion and subgroup formation and work with the meta-family, are described for the first time in the context of MFT and possibly represent mediators in the therapeutic process.The model of affect and process resonance in Plassmann's individual therapy can be applied well to the processes in the MFT and possibly represents its own effect factor through the production of episodes of secure attachment. Further research on the factors of change in the MFT is of practical relevance.