The emerging concept of social system psychotherapy is discussed. The development of this approach to psychotherapy represents a gradual evolution in clinical practices from a one-to-one interaction to a multi-person, multi-relational interaction. It also represents the development of a human psychology that has moved from individualistic to social psychology. The techniques of social system psychotherapy are cast in a new model termed the "open" model of treatment in contrast to the "closed" conventional treatment model. In the one-to-one "closed" model the assumption is that psychotherapy will effect changes in the individual to enable him to behave differently in his social fields and social networks. The open model assumes that human is significantly determined by characteristics of the social field and therapy is thus achieved via change in the social system. The two models are not competitive however, but complementary. The development of a social system psychotherapy provides a theoretical and technical base for application to many areas of mental health intervention. (Author/RSM)